Fixes #1073, #1058, #480 #1049. Provides more automation to the ScrollBarView, allowing easily implement on any view.

Terminal.Gui/Core/View.cs

@@ -741,7 +741,7 @@ namespace Terminal.Gui {
 		public void SetChildNeedsDisplay ()
 		{
 			ChildNeedsDisplay = true;
-			if (container != null)
+			if (container != null && !container.ChildNeedsDisplay)
 				container.SetChildNeedsDisplay ();
 		}
 
@@ -1342,6 +1342,7 @@ namespace Terminal.Gui {
 							// Draw the subview
 							// Use the view's bounds (view-relative; Location will always be (0,0)
 							if (view.Visible && view.Frame.Width > 0 && view.Frame.Height > 0) {
+								view.OnDrawContent (view.Bounds);
 								view.Redraw (view.Bounds);
 							}
 						}

Terminal.Gui/Views/ListView.cs

@@ -298,11 +298,6 @@ namespace Terminal.Gui {
 			Driver.SetAttribute (current);
 			Move (0, 0);
 			var f = Frame;
-			if (selected < top) {
-				top = selected;
-			} else if (selected >= top + f.Height) {
-				top = selected;
-			}
 			var item = top;
 			bool focused = HasFocus;
 			int col = allowsMarking ? 2 : 0;
@@ -477,8 +472,11 @@ namespace Terminal.Gui {
 			} else if (selected + 1 < source.Count) { //can move by down by one.
 				selected++;
 
-				if (selected >= top + Frame.Height)
+				if (selected >= top + Frame.Height) {
 					top++;
+				} else if (selected < top) {
+					top = selected;
+				}
 				OnSelectedChanged ();
 				SetNeedsDisplay ();
 			} else if (selected == 0) {
@@ -511,8 +509,11 @@ namespace Terminal.Gui {
 				if (selected > Source.Count) {
 					selected = Source.Count - 1;
 				}
-				if (selected < top)
+				if (selected < top) {
 					top = selected;
+				} else if (selected > top + Frame.Height) {
+					top = Math.Max (selected - Frame.Height + 1, 0);
+				}
 				OnSelectedChanged ();
 				SetNeedsDisplay ();
 			}
@@ -551,6 +552,26 @@ namespace Terminal.Gui {
 			return true;
 		}
 
+		/// <summary>
+		/// Scrolls the view down.
+		/// </summary>
+		/// <param name="lines">Number of lines to scroll down.</param>
+		public virtual void ScrollDown (int lines)
+		{
+			top = Math.Min (top + lines, source.Count - 1);
+			SetNeedsDisplay ();
+		}
+
+		/// <summary>
+		/// Scrolls the view up.
+		/// </summary>
+		/// <param name="lines">Number of lines to scroll up.</param>
+		public virtual void ScrollUp (int lines)
+		{
+			top = Math.Max (top - lines, 0);
+			SetNeedsDisplay ();
+		}
+
 		int lastSelectedItem = -1;
 		private bool allowsMultipleSelection = true;
 
@@ -630,10 +651,10 @@ namespace Terminal.Gui {
 			}
 
 			if (me.Flags == MouseFlags.WheeledDown) {
-				MoveDown ();
+				ScrollDown (1);
 				return true;
 			} else if (me.Flags == MouseFlags.WheeledUp) {
-				MoveUp ();
+				ScrollUp (1);
 				return true;
 			}
 
@@ -655,6 +676,8 @@ namespace Terminal.Gui {
 
 			return true;
 		}
+
+
 	}
 
 	/// <summary>

Terminal.Gui/Views/ScrollBarView.cs

@@ -0,0 +1,538 @@
+//
+// ScrollBarView.cs: ScrollBarView view.
+//
+// Authors:
+//   Miguel de Icaza ([email protected])
+//
+
+using System;
+
+namespace Terminal.Gui {
+	/// <summary>
+	/// ScrollBarViews are views that display a 1-character scrollbar, either horizontal or vertical
+	/// </summary>
+	/// <remarks>
+	/// <para>
+	///   The scrollbar is drawn to be a representation of the Size, assuming that the 
+	///   scroll position is set at Position.
+	/// </para>
+	/// <para>
+	///   If the region to display the scrollbar is larger than three characters, 
+	///   arrow indicators are drawn.
+	/// </para>
+	/// </remarks>
+	public class ScrollBarView : View {
+		bool vertical;
+		int size, position, contentOffset;
+		bool showScrollIndicator;
+		bool keepContentAlwaysInViewport = true;
+		bool autoHideScrollBars = true;
+		Dim originalHostWidth, originalHostHeight;
+		bool hosted;
+		bool showBothScrollIndicator => OtherScrollBarView != null && OtherScrollBarView.showScrollIndicator;
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Absolute"/> layout.
+		/// </summary>
+		/// <param name="rect">Frame for the scrollbar.</param>
+		public ScrollBarView (Rect rect) : this (rect, 0, 0, false) { }
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Absolute"/> layout.
+		/// </summary>
+		/// <param name="rect">Frame for the scrollbar.</param>
+		/// <param name="size">The size that this scrollbar represents. Sets the <see cref="Size"/> property.</param>
+		/// <param name="position">The position within this scrollbar. Sets the <see cref="Position"/> property.</param>
+		/// <param name="isVertical">If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal. Sets the <see cref="IsVertical"/> property.</param>
+		public ScrollBarView (Rect rect, int size, int position, bool isVertical) : base (rect)
+		{
+			Init (size, position, isVertical);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Computed"/> layout.
+		/// </summary>
+		public ScrollBarView () : this (0, 0, false) { }
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Computed"/> layout.
+		/// </summary>
+		/// <param name="size">The size that this scrollbar represents.</param>
+		/// <param name="position">The position within this scrollbar.</param>
+		/// <param name="isVertical">If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal.</param>
+		public ScrollBarView (int size, int position, bool isVertical) : base ()
+		{
+			Init (size, position, isVertical);
+		}
+
+		/// <summary>
+		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Computed"/> layout.
+		/// </summary>
+		public ScrollBarView (View host, bool isVertical) : this (0, 0, isVertical)
+		{
+			hosted = true;
+			originalHostWidth = host.Width;
+			originalHostHeight = host.Height;
+			X = isVertical ? Pos.Right(host) : Pos.Left (host);
+			Y = isVertical ? Pos.Top (host) : Pos.Bottom (host);
+			Host = host;
+			Host.SuperView.Add (this);
+			ShowScrollIndicator = true;
+			AutoHideScrollBars = true;
+		}
+
+		void Init (int size, int position, bool isVertical)
+		{
+			vertical = isVertical;
+			this.position = position;
+			this.size = size;
+			WantContinuousButtonPressed = true;
+		}
+
+		/// <summary>
+		/// If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal.
+		/// </summary>
+		public bool IsVertical {
+			get => vertical;
+			set {
+				vertical = value;
+				SetNeedsDisplay ();
+			}
+		}
+
+		/// <summary>
+		/// The size of content the scrollbar represents.
+		/// </summary>
+		/// <value>The size.</value>
+		/// <remarks>The <see cref="Size"/> is typically the size of the virtual content. E.g. when a Scrollbar is
+		/// part of a <see cref="View"/> the Size is set to the appropriate dimension of <see cref="Host"/>.</remarks>
+		public int Size {
+			get => size;
+			set {
+				size = value;
+				ShowHideScrollBars ();
+				SetNeedsDisplay ();
+			}
+		}
+
+		/// <summary>
+		/// Represents the top left/top corner coordinate that is displayed by the scrollbar.
+		/// </summary>
+		/// <value>The content offset.</value>
+		public int ContentOffset {
+			get {
+				return contentOffset;
+			}
+			set {
+				var co = -Math.Abs (value);
+				if (contentOffset != co) {
+					if (CanScroll (contentOffset - co, out int max, vertical)) {
+						if (max == contentOffset - co) {
+							contentOffset = co;
+						} else {
+							contentOffset = co + max;
+						}
+					}
+					Position = Math.Max (0, -contentOffset);
+					OnChangedPosition ();
+					SetNeedsDisplay ();
+				}
+			}
+		}
+
+		/// <summary>
+		/// This event is raised when the position on the scrollbar has changed.
+		/// </summary>
+		public event Action ChangedPosition;
+
+		/// <summary>
+		/// The position, relative to <see cref="Size"/>, to set the scrollbar at.
+		/// </summary>
+		/// <value>The position.</value>
+		public int Position {
+			get => position;
+			set {
+				position = value;
+				SetNeedsDisplay ();
+			}
+		}
+
+		/// <summary>
+		/// Get or sets the view that host this <see cref="View"/>
+		/// </summary>
+		public View Host { get; internal set; }
+
+		/// <summary>
+		/// Represent a vertical or horizontal ScrollBarView other than this.
+		/// </summary>
+		public ScrollBarView OtherScrollBarView { get; set; }
+
+		/// <summary>
+		/// Gets or sets the visibility for the vertical or horizontal scroll indicator.
+		/// </summary>
+		/// <value><c>true</c> if show vertical or horizontal scroll indicator; otherwise, <c>false</c>.</value>
+		public bool ShowScrollIndicator {
+			get => showScrollIndicator;
+			set {
+				if (value == showScrollIndicator) {
+					return;
+				}
+
+				showScrollIndicator = value;
+				SetNeedsLayout ();
+				if (value) {
+					Visible = true;
+				} else {
+					Visible = false;
+				}
+				Width = vertical ? 1 : Dim.Width (Host);
+				Height = vertical ? Dim.Height (Host) : 1;
+				if (vertical) {
+					Host.Width = showScrollIndicator ? originalHostWidth - 1 : originalHostWidth;
+				} else {
+					Host.Height = showScrollIndicator ? originalHostHeight - 1 : originalHostHeight;
+				}
+			}
+		}
+
+		/// <summary>
+		/// Get or sets if the view-port is kept always visible in the area of this <see cref="ScrollBarView"/>
+		/// </summary>
+		public bool KeepContentAlwaysInViewport {
+			get { return keepContentAlwaysInViewport; }
+			set {
+				if (keepContentAlwaysInViewport != value) {
+					keepContentAlwaysInViewport = value;
+					int co = 0;
+					if (value && !vertical && -contentOffset + Host.Bounds.Width > size) {
+						co = size - Host.Bounds.Width + (showBothScrollIndicator ? 1 : 0);
+					}
+					if (value && vertical && -contentOffset + Host.Bounds.Height > size) {
+						co = size - Host.Bounds.Height + (showBothScrollIndicator ? 1 : 0);
+					}
+					if (co != 0) {
+						ContentOffset = co;
+					}
+					if (OtherScrollBarView != null && OtherScrollBarView.keepContentAlwaysInViewport != value) {
+						OtherScrollBarView.KeepContentAlwaysInViewport = value;
+					}
+				}
+			}
+		}
+
+		/// <summary>
+		/// If true the vertical/horizontal scroll bars won't be showed if it's not needed.
+		/// </summary>
+		public bool AutoHideScrollBars {
+			get => autoHideScrollBars;
+			set {
+				if (autoHideScrollBars != value) {
+					autoHideScrollBars = value;
+					SetNeedsDisplay ();
+				}
+			}
+		}
+
+		void SetPosition (int newPos)
+		{
+			Position = newPos;
+			ContentOffset = Position;
+			OnChangedPosition ();
+		}
+
+		/// <summary>
+		/// Virtual method to invoke the <see cref="ChangedPosition"/> action event.
+		/// </summary>
+		public virtual void OnChangedPosition ()
+		{
+			ChangedPosition?.Invoke ();
+		}
+
+		internal bool pending;
+
+		void ShowHideScrollBars ()
+		{
+			if (!hosted || !autoHideScrollBars) {
+				return;
+			}
+
+			if (vertical) {
+				if (Host.Bounds.Height == 0 || Host.Bounds.Height > size) {
+					if (showScrollIndicator) {
+						ShowScrollIndicator = false;
+					}
+				} else if (Host.Bounds.Height > 0 && Host.Bounds.Height == size) {
+					pending = true;
+				} else if (!showScrollIndicator) {
+					ShowScrollIndicator = true;
+				}
+			} else {
+				if (Host.Bounds.Width == 0 || Host.Bounds.Width > size) {
+					if (showScrollIndicator) {
+						ShowScrollIndicator = false;
+					}
+				} else if (Host.Bounds.Width > 0 && Host.Bounds.Width == size && OtherScrollBarView.pending) {
+					if (showScrollIndicator) {
+						ShowScrollIndicator = false;
+					}
+					if (showBothScrollIndicator) {
+						OtherScrollBarView.showScrollIndicator = false;
+					}
+				} else {
+					if (OtherScrollBarView.pending) {
+						if (!showBothScrollIndicator) {
+							OtherScrollBarView.showScrollIndicator = true;
+						}
+					}
+					if (!showScrollIndicator) {
+						ShowScrollIndicator = true;
+					}
+				}
+			}
+		}
+
+		int posTopTee;
+		int posLeftTee;
+		int posBottomTee;
+		int posRightTee;
+
+		///<inheritdoc/>
+		public override void Redraw (Rect region)
+		{
+			if (ColorScheme == null || Size == 0) {
+				return;
+			}
+
+			Driver.SetAttribute (ColorScheme.Normal);
+
+			if ((vertical && Bounds.Height == 0) || (!vertical && Bounds.Width == 0)) {
+				return;
+			}
+
+			if (vertical) {
+				if (region.Right < Bounds.Width - 1) {
+					return;
+				}
+
+				var col = Bounds.Width - 1;
+				var bh = Bounds.Height;
+				Rune special;
+
+				if (bh < 4) {
+					var by1 = position * bh / Size;
+					var by2 = (position + bh) * bh / Size;
+
+					Move (col, 0);
+					if (Bounds.Height == 1) {
+						Driver.AddRune (Driver.Diamond);
+					} else {
+						Driver.AddRune (Driver.UpArrow);
+					}
+					if (Bounds.Height == 3) {
+						Move (col, 1);
+						Driver.AddRune (Driver.Diamond);
+					}
+					if (Bounds.Height > 1) {
+						Move (col, Bounds.Height - 1);
+						Driver.AddRune (Driver.DownArrow);
+					}
+				} else {
+					bh -= 2;
+					var by1 = position * bh / Size;
+					var by2 = KeepContentAlwaysInViewport ? Math.Min (((position + bh) * bh / Size) + 1, bh - 1) : (position + bh) * bh / Size;
+					if (KeepContentAlwaysInViewport && by1 == by2) {
+						by1 = Math.Max (by1 - 1, 0);
+					}
+
+					Move (col, 0);
+					Driver.AddRune (Driver.UpArrow);
+					Move (col, Bounds.Height - 1);
+					Driver.AddRune (Driver.DownArrow);
+
+					bool hasTopTee = false;
+					bool hasDiamond = false;
+					bool hasBottomTee = false;
+					for (int y = 0; y < bh; y++) {
+						Move (col, y + 1);
+						if ((y < by1 || y > by2) && ((position > 0 && !hasTopTee) || (hasTopTee && hasBottomTee))) {
+							special = Driver.Stipple;
+						} else {
+							if (y != by2 && y > 1 && by2 - by1 == 0 && by1 < bh - 1 && hasTopTee && !hasDiamond) {
+								hasDiamond = true;
+								special = Driver.Diamond;
+							} else {
+								if (y == by1 && !hasTopTee) {
+									hasTopTee = true;
+									posTopTee = y;
+									special = Driver.TopTee;
+								} else if ((position == 0 && y == bh - 1 || y >= by2 || by2 == 0) && !hasBottomTee) {
+									hasBottomTee = true;
+									posBottomTee = y;
+									special = Driver.BottomTee;
+								} else {
+									special = Driver.VLine;
+								}
+							}
+						}
+						Driver.AddRune (special);
+					}
+					if (!hasTopTee) {
+						Move (col, Bounds.Height - 2);
+						Driver.AddRune (Driver.TopTee);
+					}
+				}
+			} else {
+				if (region.Bottom < Bounds.Height - 1) {
+					return;
+				}
+
+				var row = Bounds.Height - 1;
+				var bw = Bounds.Width;
+				Rune special;
+
+				if (bw < 4) {
+					var bx1 = position * bw / Size;
+					var bx2 = (position + bw) * bw / Size;
+
+					Move (0, row);
+					Driver.AddRune (Driver.LeftArrow);
+					Driver.AddRune (Driver.RightArrow);
+				} else {
+					bw -= 2;
+					var bx1 = position * bw / Size;
+					var bx2 = KeepContentAlwaysInViewport ? Math.Min (((position + bw) * bw / Size) + 1, bw - 1) : (position + bw) * bw / Size;
+					if (KeepContentAlwaysInViewport && bx1 == bx2) {
+						bx1 = Math.Max (bx1 - 1, 0);
+					}
+
+					Move (0, row);
+					Driver.AddRune (Driver.LeftArrow);
+
+					bool hasLeftTee = false;
+					bool hasDiamond = false;
+					bool hasRightTee = false;
+					for (int x = 0; x < bw; x++) {
+						if ((x < bx1 || x >= bx2 + 1) && ((position > 0 && !hasLeftTee) || (hasLeftTee && hasRightTee))) {
+							special = Driver.Stipple;
+						} else {
+							if (x != bx2 && x > 1 && bx2 - bx1 == 0 && bx1 < bw - 1 && hasLeftTee && !hasDiamond) {
+								hasDiamond = true;
+								special = Driver.Diamond;
+							} else {
+								if (x == bx1 && !hasLeftTee) {
+									hasLeftTee = true;
+									posLeftTee = x;
+									special = Driver.LeftTee;
+								} else if ((position == 0 && x == bw - 1 || x >= bx2 || bx2 == 0) && !hasRightTee) {
+									hasRightTee = true;
+									posRightTee = x;
+									special = Driver.RightTee;
+								} else {
+									special = Driver.HLine;
+								}
+							}
+						}
+						Driver.AddRune (special);
+					}
+					if (!hasLeftTee) {
+						Move (Bounds.Width -2, row);
+						Driver.AddRune (Driver.LeftTee);
+					}
+
+					Driver.AddRune (Driver.RightArrow);
+				}
+			}
+		}
+
+		int lastLocation = -1;
+
+		///<inheritdoc/>
+		public override bool MouseEvent (MouseEvent me)
+		{
+			if (me.Flags != MouseFlags.Button1Pressed && me.Flags != MouseFlags.Button1Clicked &&
+				!me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
+				return false;
+			}
+
+			if (!me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
+				lastLocation = -1;
+			}
+
+			int location = vertical ? me.Y : me.X;
+			int barsize = vertical ? Bounds.Height : Bounds.Width;
+			int posTopLeftTee = vertical ? posTopTee : posLeftTee;
+			int posBottomRightTee = vertical ? posBottomTee : posRightTee;
+
+			barsize -= 2;
+			var pos = Position;
+			if (location == 0) {
+				if (pos > 0) {
+					SetPosition (pos - 1);
+				}
+			} else if (location == barsize + 1) {
+				if (CanScroll (1, out _, vertical)) {
+					SetPosition (pos + 1);
+				}
+			} else if (location > 0 && location < barsize + 1) {
+				var b1 = pos * barsize / Size;
+				var b2 = KeepContentAlwaysInViewport ? Math.Min (((pos + barsize) * barsize / Size) + 1, barsize - 1) : (pos + barsize) * barsize / Size;
+				if (KeepContentAlwaysInViewport && b1 == b2) {
+					b1 = Math.Max (b1 - 1, 0);
+				}
+
+				if (location > b1 && location <= b2 + 1) {
+					if (me.Flags == MouseFlags.Button1Pressed || me.Flags == MouseFlags.Button1Clicked) {
+						if (location == 1) {
+							SetPosition (0);
+						} else if (location == barsize) {
+							CanScroll (Size - pos, out int nv, vertical);
+							if (nv > 0) {
+								SetPosition (Math.Min (pos + nv, Size));
+							}
+						}
+					} else if (me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
+						var mb = (b2 - b1) / 2;
+						var ml = mb + b1 + (mb == 0 ? 1 : 0);
+						if ((location >= b1 && location <= ml) || (location < lastLocation && lastLocation > -1)) {
+							lastLocation = location;
+							var np = b1 * Size / barsize;
+							SetPosition (np);
+						} else if (location > lastLocation) {
+							var np = location * Size / barsize;
+							CanScroll (np - pos, out int nv, vertical);
+							if (nv > 0) {
+								SetPosition (pos + nv);
+							}
+						}
+					}
+				} else {
+					if (location >= b2 + 1 && location > posTopLeftTee && location > b1 && location > posBottomRightTee && posBottomRightTee > 0) {
+						CanScroll (location, out int nv, vertical);
+						if (nv > 0) {
+							SetPosition (Math.Min (pos + nv, Size));
+						}
+					} else if (location <= b1) {
+						SetPosition (Math.Max (pos - barsize - location, 0));
+					}
+				}
+			}
+
+			return true;
+		}
+
+		internal bool CanScroll (int n, out int max, bool isVertical = false)
+		{
+			var size = isVertical ?
+				(KeepContentAlwaysInViewport ? Host.Bounds.Height + (showBothScrollIndicator ? -2 : -1) : 0) :
+				(KeepContentAlwaysInViewport ? Host.Bounds.Width + (showBothScrollIndicator ? -2 : -1) : 0);
+			var cSize = -Size;
+			var cOffSet = contentOffset;
+			var newSize = Math.Max (cSize, cOffSet - n);
+			max = cSize < newSize - size ? n : -cSize + (cOffSet - size) - 1;
+			if (cSize < newSize - size) {
+				return true;
+			}
+			return false;
+		}
+	}
+}

Terminal.Gui/Views/ScrollView.cs

@@ -1,5 +1,5 @@
 //
-// ScrollView.cs: ScrollView and ScrollBarView views.
+// ScrollView.cs: ScrollView view.
 //
 // Authors:
 //   Miguel de Icaza ([email protected])
@@ -15,345 +15,6 @@ using System;
 using System.Reflection;
 
 namespace Terminal.Gui {
-	/// <summary>
-	/// ScrollBarViews are views that display a 1-character scrollbar, either horizontal or vertical
-	/// </summary>
-	/// <remarks>
-	/// <para>
-	///   The scrollbar is drawn to be a representation of the Size, assuming that the 
-	///   scroll position is set at Position.
-	/// </para>
-	/// <para>
-	///   If the region to display the scrollbar is larger than three characters, 
-	///   arrow indicators are drawn.
-	/// </para>
-	/// </remarks>
-	public class ScrollBarView : View {
-		bool vertical = false;
-		int size = 0, position = 0;
-
-		/// <summary>
-		/// If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal.
-		/// </summary>
-		public bool IsVertical {
-			get => vertical;
-			set {
-				vertical = value;
-				SetNeedsDisplay ();
-			}
-		}
-
-		/// <summary>
-		/// The size of content the scrollbar represents. 
-		/// </summary>
-		/// <value>The size.</value>
-		/// <remarks>The <see cref="Size"/> is typically the size of the virtual content. E.g. when a Scrollbar is
-		/// part of a <see cref="ScrollView"/> the Size is set to the appropriate dimension of <see cref="ScrollView.ContentSize"/>.</remarks>
-		public int Size {
-			get => size;
-			set {
-				size = value;
-				SetNeedsDisplay ();
-			}
-		}
-
-		/// <summary>
-		/// This event is raised when the position on the scrollbar has changed.
-		/// </summary>
-		public event Action ChangedPosition;
-
-		/// <summary>
-		/// The position, relative to <see cref="Size"/>, to set the scrollbar at.
-		/// </summary>
-		/// <value>The position.</value>
-		public int Position {
-			get => position;
-			set {
-				position = value;
-				SetNeedsDisplay ();
-			}
-		}
-
-		/// <summary>
-		/// Get or sets the view that host this <see cref="ScrollView"/>
-		/// </summary>
-		public ScrollView Host { get; internal set; }
-
-		void SetPosition (int newPos)
-		{
-			Position = newPos;
-			ChangedPosition?.Invoke ();
-		}
-
-		/// <summary>
-		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Absolute"/> layout.
-		/// </summary>
-		/// <param name="rect">Frame for the scrollbar.</param>
-		public ScrollBarView (Rect rect) : this (rect, 0, 0, false) { }
-
-		/// <summary>
-		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Absolute"/> layout.
-		/// </summary>
-		/// <param name="rect">Frame for the scrollbar.</param>
-		/// <param name="size">The size that this scrollbar represents. Sets the <see cref="Size"/> property.</param>
-		/// <param name="position">The position within this scrollbar. Sets the <see cref="Position"/> property.</param>
-		/// <param name="isVertical">If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal. Sets the <see cref="IsVertical"/> property.</param>
-		public ScrollBarView (Rect rect, int size, int position, bool isVertical) : base (rect)
-		{
-			Init (size, position, isVertical);
-		}
-
-		/// <summary>
-		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Computed"/> layout.
-		/// </summary>
-		public ScrollBarView () : this (0, 0, false) { }
-
-		/// <summary>
-		/// Initializes a new instance of the <see cref="Gui.ScrollBarView"/> class using <see cref="LayoutStyle.Computed"/> layout.
-		/// </summary>
-		/// <param name="size">The size that this scrollbar represents.</param>
-		/// <param name="position">The position within this scrollbar.</param>
-		/// <param name="isVertical">If set to <c>true</c> this is a vertical scrollbar, otherwise, the scrollbar is horizontal.</param>
-		public ScrollBarView (int size, int position, bool isVertical) : base ()
-		{
-			Init (size, position, isVertical);
-		}
-
-		void Init (int size, int position, bool isVertical)
-		{
-			vertical = isVertical;
-			this.position = position;
-			this.size = size;
-			WantContinuousButtonPressed = true;
-		}
-
-		int posTopTee;
-		int posLeftTee;
-		int posBottomTee;
-		int posRightTee;
-
-		///<inheritdoc/>
-		public override void Redraw (Rect region)
-		{
-			if (ColorScheme == null || Size == 0)
-				return;
-
-			Driver.SetAttribute (ColorScheme.Normal);
-
-			if (Bounds.Height == 0) {
-				return;
-			}
-
-			if (vertical) {
-				if (region.Right < Bounds.Width - 1)
-					return;
-
-				var col = Bounds.Width - 1;
-				var bh = Bounds.Height;
-				Rune special;
-
-				if (bh < 4) {
-					var by1 = position * bh / Size;
-					var by2 = (position + bh) * bh / Size;
-
-					Move (col, 0);
-					if (Bounds.Height == 1) {
-						Driver.AddRune (Driver.Diamond);
-					} else {
-						Driver.AddRune (Driver.UpArrow);
-					}
-					if (Bounds.Height == 3) {
-						Move (col, 1);
-						Driver.AddRune (Driver.Diamond);
-					}
-					if (Bounds.Height > 1) {
-						Move (col, Bounds.Height - 1);
-						Driver.AddRune (Driver.DownArrow);
-					}
-				} else {
-					bh -= 2;
-					var by1 = position * bh / Size;
-					var by2 = Host.KeepContentAlwaysInViewport ? Math.Min (((position + bh) * bh / Size) + 1, bh - 1) : (position + bh) * bh / Size;
-					if (Host.KeepContentAlwaysInViewport && by1 == by2) {
-						by1 = Math.Max (by1 - 1, 0);
-					}
-
-					Move (col, 0);
-					Driver.AddRune (Driver.UpArrow);
-					Move (col, Bounds.Height - 1);
-					Driver.AddRune (Driver.DownArrow);
-
-					bool hasTopTee = false;
-					bool hasDiamond = false;
-					bool hasBottomTee = false;
-					for (int y = 0; y < bh; y++) {
-						Move (col, y + 1);
-						if ((y < by1 || y > by2) && ((position > 0 && !hasTopTee) || (hasTopTee && hasBottomTee))) {
-							special = Driver.Stipple;
-						} else {
-							if (y != by2 && y > 1 && by2 - by1 == 0 && by1 < bh - 1 && hasTopTee && !hasDiamond) {
-								hasDiamond = true;
-								special = Driver.Diamond;
-							} else {
-								if (y == by1 && !hasTopTee) {
-									hasTopTee = true;
-									posTopTee = y;
-									special = Driver.TopTee;
-								} else if ((position == 0 && y == bh - 1 || y >= by2 || by2 == 0) && !hasBottomTee) {
-									hasBottomTee = true;
-									posBottomTee = y;
-									special = Driver.BottomTee;
-								} else {
-									special = Driver.VLine;
-								}
-							}
-						}
-						Driver.AddRune (special);
-					}
-					if (!hasTopTee) {
-						Move (col, Bounds.Height - 2);
-						Driver.AddRune (Driver.TopTee);
-					}
-				}
-			} else {
-				if (region.Bottom < Bounds.Height - 1)
-					return;
-
-				var row = Bounds.Height - 1;
-				var bw = Bounds.Width;
-				Rune special;
-
-				if (bw < 4) {
-					var bx1 = position * bw / Size;
-					var bx2 = (position + bw) * bw / Size;
-
-					Move (0, row);
-					Driver.AddRune (Driver.LeftArrow);
-					Driver.AddRune (Driver.RightArrow);
-				} else {
-					bw -= 2;
-					var bx1 = position * bw / Size;
-					var bx2 = Host.KeepContentAlwaysInViewport ? Math.Min (((position + bw) * bw / Size) + 1, bw - 1) : (position + bw) * bw / Size;
-					if (Host.KeepContentAlwaysInViewport && bx1 == bx2) {
-						bx1 = Math.Max (bx1 - 1, 0);
-					}
-
-					Move (0, row);
-					Driver.AddRune (Driver.LeftArrow);
-
-					bool hasLeftTee = false;
-					bool hasDiamond = false;
-					bool hasRightTee = false;
-					for (int x = 0; x < bw; x++) {
-						if ((x < bx1 || x >= bx2 + 1) && ((position > 0 && !hasLeftTee) || (hasLeftTee && hasRightTee))) {
-							special = Driver.Stipple;
-						} else {
-							if (x != bx2 && x > 1 && bx2 - bx1 == 0 && bx1 < bw - 1 && hasLeftTee && !hasDiamond) {
-								hasDiamond = true;
-								special = Driver.Diamond;
-							} else {
-								if (x == bx1 && !hasLeftTee) {
-									hasLeftTee = true;
-									posLeftTee = x;
-									special = Driver.LeftTee;
-								} else if ((position == 0 && x == bw - 1 || x >= bx2 || bx2 == 0) && !hasRightTee) {
-									hasRightTee = true;
-									posRightTee = x;
-									special = Driver.RightTee;
-								} else {
-									special = Driver.HLine;
-								}
-							}
-						}
-						Driver.AddRune (special);
-					}
-					if (!hasLeftTee) {
-						Move (Bounds.Width -2, row);
-						Driver.AddRune (Driver.LeftTee);
-					}
-
-					Driver.AddRune (Driver.RightArrow);
-				}
-			}
-		}
-
-		int lastLocation = -1;
-
-		///<inheritdoc/>
-		public override bool MouseEvent (MouseEvent me)
-		{
-			if (me.Flags != MouseFlags.Button1Pressed && me.Flags != MouseFlags.Button1Clicked &&
-				!me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
-				return false;
-			}
-
-			if (!me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
-				lastLocation = -1;
-			}
-
-			int location = vertical ? me.Y : me.X;
-			int barsize = vertical ? Bounds.Height : Bounds.Width;
-			int posTopLeftTee = vertical ? posTopTee : posLeftTee;
-			int posBottomRightTee = vertical ? posBottomTee : posRightTee;
-
-			barsize -= 2;
-			var pos = Position;
-			if (location == 0) {
-				if (pos > 0) {
-					SetPosition (pos - 1);
-				}
-			} else if (location == barsize + 1) {
-				if (Host.CanScroll (1, out _, vertical)) {
-					SetPosition (pos + 1);
-				}
-			} else if (location > 0 && location < barsize + 1) {
-				var b1 = pos * barsize / Size;
-				var b2 = Host.KeepContentAlwaysInViewport ? Math.Min (((pos + barsize) * barsize / Size) + 1, barsize - 1) : (pos + barsize) * barsize / Size;
-				if (Host.KeepContentAlwaysInViewport && b1 == b2) {
-					b1 = Math.Max (b1 - 1, 0);
-				}
-
-				if (location > b1 && location <= b2 + 1) {
-					if (me.Flags == MouseFlags.Button1Pressed || me.Flags == MouseFlags.Button1Clicked) {
-						if (location == 1) {
-							SetPosition (0);
-						} else if (location == barsize) {
-							Host.CanScroll (Size - pos, out int nv, vertical);
-							if (nv > 0) {
-								SetPosition (Math.Min (pos + nv, Size));
-							}
-						}
-					} else if (me.Flags.HasFlag (MouseFlags.Button1Pressed | MouseFlags.ReportMousePosition)) {
-						var mb = (b2 - b1) / 2;
-						var ml = mb + b1 + (mb == 0 ? 1 : 0);
-						if ((location >= b1 && location <= ml) || (location < lastLocation && lastLocation > -1)) {
-							lastLocation = location;
-							var np = b1 * Size / barsize;
-							SetPosition (np);
-						} else if (location > lastLocation) {
-							var np = location * Size / barsize;
-							Host.CanScroll (np - pos, out int nv, vertical);
-							if (nv > 0) {
-								SetPosition (pos + nv);
-							}
-						}
-					}
-				} else {
-					if (location >= b2 + 1 && location > posTopLeftTee && location > b1 && location > posBottomRightTee && posBottomRightTee > 0) {
-						Host.CanScroll (location, out int nv, vertical);
-						if (nv > 0) {
-							SetPosition (Math.Min (pos + nv, Size));
-						}
-					} else if (location <= b1) {
-						SetPosition (Math.Max (pos - barsize - location, 0));
-					}
-				}
-			}
-
-			return true;
-		}
-	}
-
 	/// <summary>
 	/// Scrollviews are views that present a window into a virtual space where subviews are added.  Similar to the iOS UIScrollView.
 	/// </summary>
@@ -483,6 +144,8 @@ namespace Terminal.Gui {
 			set {
 				if (keepContentAlwaysInViewport != value) {
 					keepContentAlwaysInViewport = value;
+					vertical.OtherScrollBarView.KeepContentAlwaysInViewport = value;
+					horizontal.OtherScrollBarView.KeepContentAlwaysInViewport = value;
 					Point p = default;
 					if (value && -contentOffset.X + Bounds.Width > contentSize.Width) {
 						p = new Point (contentSize.Width - Bounds.Width + (showVerticalScrollIndicator ? 1 : 0), -contentOffset.Y);
@@ -540,17 +203,21 @@ namespace Terminal.Gui {
 		public bool ShowHorizontalScrollIndicator {
 			get => showHorizontalScrollIndicator;
 			set {
-				if (value == showHorizontalScrollIndicator)
+				if (value == showHorizontalScrollIndicator) {
 					return;
+				}
 
 				showHorizontalScrollIndicator = value;
 				SetNeedsLayout ();
 				if (value) {
 					base.Add (horizontal);
+					horizontal.OtherScrollBarView = vertical;
+					horizontal.OtherScrollBarView.ShowScrollIndicator = value;
 					horizontal.MouseEnter += View_MouseEnter;
 					horizontal.MouseLeave += View_MouseLeave;
 				} else {
-					Remove (horizontal);
+					base.Remove (horizontal);
+					horizontal.OtherScrollBarView = null;
 					horizontal.MouseEnter -= View_MouseEnter;
 					horizontal.MouseLeave -= View_MouseLeave;
 				}
@@ -575,17 +242,21 @@ namespace Terminal.Gui {
 		public bool ShowVerticalScrollIndicator {
 			get => showVerticalScrollIndicator;
 			set {
-				if (value == showVerticalScrollIndicator)
+				if (value == showVerticalScrollIndicator) {
 					return;
+				}
 
 				showVerticalScrollIndicator = value;
 				SetNeedsLayout ();
 				if (value) {
 					base.Add (vertical);
+					vertical.OtherScrollBarView = horizontal;
+					vertical.OtherScrollBarView.ShowScrollIndicator = value;
 					vertical.MouseEnter += View_MouseEnter;
 					vertical.MouseLeave += View_MouseLeave;
 				} else {
 					Remove (vertical);
+					vertical.OtherScrollBarView = null;
 					vertical.MouseEnter -= View_MouseEnter;
 					vertical.MouseLeave -= View_MouseLeave;
 				}
@@ -739,7 +410,7 @@ namespace Terminal.Gui {
 		/// <param name="lines">Number of lines to scroll.</param>
 		public bool ScrollDown (int lines)
 		{
-			if (CanScroll (lines, out _, true)) {
+			if (vertical.CanScroll (lines, out _, true)) {
 				ContentOffset = new Point (contentOffset.X, contentOffset.Y - lines);
 				return true;
 			}
@@ -753,28 +424,13 @@ namespace Terminal.Gui {
 		/// <param name="cols">Number of columns to scroll by.</param>
 		public bool ScrollRight (int cols)
 		{
-			if (CanScroll (cols, out _)) {
+			if (horizontal.CanScroll (cols, out _)) {
 				ContentOffset = new Point (contentOffset.X - cols, contentOffset.Y);
 				return true;
 			}
 			return false;
 		}
 
-		internal bool CanScroll (int n, out int max, bool isVertical = false)
-		{
-			var size = isVertical ?
-				(KeepContentAlwaysInViewport ? Bounds.Height + (showHorizontalScrollIndicator ? -2 : -1) : 0) :
-				(KeepContentAlwaysInViewport ? Bounds.Width + (showVerticalScrollIndicator ? -2 : -1) : 0);
-			var cSize = isVertical ? -contentSize.Height : -contentSize.Width;
-			var cOffSet = isVertical ? contentOffset.Y : contentOffset.X;
-			var newSize = Math.Max (cSize, cOffSet - n);
-			max = cSize < newSize - size ? n : -cSize + (cOffSet - size) - 1;
-			if (cSize < newSize - size) {
-				return true;
-			}
-			return false;
-		}
-
 		///<inheritdoc/>
 		public override bool ProcessKey (KeyEvent kb)
 		{

Terminal.Gui/Views/TextView.cs

@@ -460,6 +460,26 @@ namespace Terminal.Gui {
 			}
 		}
 
+		/// <summary>
+		/// Gets or sets the top row.
+		/// </summary>
+		public int TopRow { get => topRow; set => topRow = Math.Max (Math.Min (value, Lines - 1), 0); }
+
+		/// <summary>
+		/// Gets or sets the left column.
+		/// </summary>
+		public int LeftColumn { get => leftColumn; set => leftColumn = Math.Max (Math.Min (value, Maxlength - 1), 0); }
+
+		/// <summary>
+		/// Gets the maximum visible length line.
+		/// </summary>
+		public int Maxlength => model.GetMaxVisibleLine (topRow, topRow + Frame.Height);
+
+		/// <summary>
+		/// Gets the  number of lines.
+		/// </summary>
+		public int Lines => model.Count;
+
 		/// <summary>
 		/// Loads the contents of the file into the  <see cref="TextView"/>.
 		/// </summary>
@@ -875,10 +895,10 @@ namespace Terminal.Gui {
 				idx = 0;
 			}
 			if (isRow) {
-				topRow = idx > model.Count - 1 ? model.Count - 1 : idx;
+				topRow = Math.Max (idx > model.Count - 1 ? model.Count - 1 : idx, 0);
 			} else {
 				var maxlength = model.GetMaxVisibleLine (topRow, topRow + Frame.Height);
-				leftColumn = idx > maxlength - 1 ? maxlength - 1 : idx;
+				leftColumn = Math.Max (idx > maxlength - 1 ? maxlength - 1 : idx, 0);
 			}
 			SetNeedsDisplay ();
 		}
@@ -1374,7 +1394,7 @@ namespace Terminal.Gui {
 
 			if (ev.Flags == MouseFlags.Button1Clicked) {
 				if (model.Count > 0) {
-					var maxCursorPositionableLine = (model.Count - 1) - topRow;
+					var maxCursorPositionableLine = Math.Max ((model.Count - 1) - topRow, 0);
 					if (ev.Y > maxCursorPositionableLine) {
 						currentRow = maxCursorPositionableLine;
 					} else {

UICatalog/Scenarios/Editor.cs

@@ -13,6 +13,7 @@ namespace UICatalog {
 		private string _fileName = "demo.txt";
 		private TextView _textView;
 		private bool _saved = true;
+		private ScrollBarView _vertical;
 
 		public override void Init (Toplevel top, ColorScheme colorScheme)
 		{
@@ -35,6 +36,7 @@ namespace UICatalog {
 					new MenuItem ("C_ut", "", () => Cut()),
 					new MenuItem ("_Paste", "", () => Paste())
 				}),
+				new MenuBarItem ("_ScrollBarView", CreateKeepChecked ())
 			});
 			Top.Add (menu);
 
@@ -67,6 +69,41 @@ namespace UICatalog {
 			LoadFile ();
 
 			Win.Add (_textView);
+
+			_vertical = new ScrollBarView (_textView, true);
+			var horizontal = new ScrollBarView (_textView, false);
+			_vertical.OtherScrollBarView = horizontal;
+			horizontal.OtherScrollBarView = _vertical;
+
+			_vertical.ChangedPosition += () => {
+				_textView.TopRow = _vertical.Position;
+				if (_textView.TopRow != _vertical.Position) {
+					_vertical.Position = _textView.TopRow;
+				}
+				_textView.SetNeedsDisplay ();
+			};
+
+			horizontal.ChangedPosition += () => {
+				_textView.LeftColumn = horizontal.Position;
+				if (_textView.LeftColumn != horizontal.Position) {
+					horizontal.Position = _textView.LeftColumn;
+				}
+				_textView.SetNeedsDisplay ();
+			};
+
+			_textView.DrawContent += (e) => {
+				_vertical.Size = _textView.Lines - 1;
+				_vertical.ContentOffset = _textView.TopRow;
+				horizontal.Size = _textView.Maxlength - 1;
+				horizontal.ContentOffset = _textView.LeftColumn;
+				_vertical.ColorScheme = horizontal.ColorScheme = _textView.ColorScheme;
+				if (_vertical.ShowScrollIndicator) {
+					_vertical.Redraw (e);
+				}
+				if (horizontal.ShowScrollIndicator) {
+					horizontal.Redraw (e);
+				}
+			};
 		}
 
 		public override void Setup ()
@@ -145,11 +182,25 @@ namespace UICatalog {
 			sb.Append ("Hello world.\n");
 			sb.Append ("This is a test of the Emergency Broadcast System.\n");
 
+			for (int i = 0; i < 40; i++) {
+				sb.Append ("This is a test with a very long line and many lines to test the ScrollViewBar against the TextView.\n");
+			}
 			var sw = System.IO.File.CreateText (fileName);
 			sw.Write (sb.ToString ());
 			sw.Close ();
 		}
 
+		private MenuItem [] CreateKeepChecked ()
+		{
+			var item = new MenuItem ();
+			item.Title = "Keep Content Always In Viewport";
+			item.CheckType |= MenuItemCheckStyle.Checked;
+			item.Checked = true;
+			item.Action += () => _vertical.KeepContentAlwaysInViewport = item.Checked = !item.Checked;
+
+			return new MenuItem [] { item };
+		}
+
 		public override void Run ()
 		{
 			base.Run ();

UICatalog/Scenarios/ListViewWithSelection.cs

@@ -6,7 +6,7 @@ using System.Linq;
 using Terminal.Gui;
 
 namespace UICatalog {
-	[ScenarioMetadata (Name: "List View With Selection", Description: "ListView with colunns and selection")]
+	[ScenarioMetadata (Name: "List View With Selection", Description: "ListView with columns and selection")]
 	[ScenarioCategory ("Controls")]
 	class ListViewWithSelection : Scenario {
 
@@ -55,9 +55,34 @@ namespace UICatalog {
 			};
 			Win.Add (_listView);
 
-			
+			var vertical = new ScrollBarView (_listView, true);
+
+			vertical.ChangedPosition += () => {
+				_listView.TopItem = vertical.Position;
+				if (_listView.TopItem != vertical.Position) {
+					vertical.Position = _listView.TopItem;
+				}
+				_listView.SetNeedsDisplay ();
+			};
+
+			_listView.DrawContent += (e) => {
+				vertical.Size = _listView.Source.Count;
+				vertical.ContentOffset = _listView.TopItem;
+				vertical.ColorScheme = _listView.ColorScheme;
+				if (vertical.ShowScrollIndicator) {
+					vertical.Redraw (e);
+				}
+			};
+
 			_listView.SetSource (_scenarios);
 
+			var k = "Keep Content Always In Viewport";
+			var keepCheckBox = new CheckBox (k, vertical.AutoHideScrollBars) {
+				X = Pos.AnchorEnd (k.Length + 3),
+				Y = 0,
+			};
+			keepCheckBox.Toggled += (_) => vertical.KeepContentAlwaysInViewport = keepCheckBox.Checked;
+			Win.Add (keepCheckBox);
 		}
 
 		private void _customRenderCB_Toggled (bool prev)
@@ -84,7 +109,7 @@ namespace UICatalog {
 			Win.SetNeedsDisplay ();
 		}
 
-		// This is basicaly the same implementation used by the UICatalog main window
+		// This is basically the same implementation used by the UICatalog main window
 		internal class ScenarioListDataSource : IListDataSource {
 			int _nameColumnWidth = 30;
 			private List<Type> scenarios;