Scrolling: Made mouse-wheel scrolling lock the underlying window until the mouse is moved again or until a short delay expires (2 seconds). This allow uninterrupted scroll even if child windows are passing under the mouse cursor. (#2604)

docs/CHANGELOG.txt

@@ -53,7 +53,6 @@ Other Changes:
   any more. Forwarding can still be disabled by setting ImGuiWindowFlags_NoInputs. (amend #1502, #1380).
 - Window: Fixed old SetWindowFontScale() api value from not being inherited by child window. Added
   comments about the right way to scale your UI (load a font at the right side, rebuild atlas, scale style).
-- Scrollbar: Avoid overlapping the opposite side when window (often a child window) is forcibly too small.
 - Combo: Hide arrow when there's not enough space even for the square button.
 - TabBar: Fixed unfocused tab bar separator color (was using ImGuiCol_Tab, should use ImGuiCol_TabUnfocusedActive).
 - Columns: Fixed a regression from 1.71 where the right-side of the contents rectangle within each column
@@ -67,12 +66,16 @@ Other Changes:
 - InputTextMultiline: Fixed vertical scrolling tracking glitch.
 - Word-wrapping: Fixed overzealous word-wrapping when glyph edge lands exactly on the limit. Because
   of this, auto-fitting exactly unwrapped text would make it wrap. (fixes initial 1.15 commit, 78645a7d).
+- Scrolling: Made mouse-wheel scrolling lock the underlying window until the mouse is moved again or
+  until a short delay expires (2 seconds). This allow uninterrupted scroll even if child windows are
+  passing under the mouse cursor. (#2604)
 - Scrolling: Made it possible for mouse wheel and navigation-triggered scrolling to override a call to
   SetScrollX()/SetScrollY(), making it possible to use a simpler stateless pattern for auto-scrolling:
      // (Submit items..)
      if (ImGui::GetScrollY() >= ImGui::GetScrollMaxY())   // Keep scrolling at the bottom if already
          ImGui::SetScrollHereY(1.0f);
 - Scrolling: Added SetScrollHereX(), SetScrollFromPosX() for completeness. (#1580) [@kevreco]
+- Scrollbar: Avoid overlapping the opposite side when window (often a child window) is forcibly too small.
 - Style: Attenuated default opacity of ImGuiCol_Separator in Classic and Light styles.
 - Style: Added style.ColorButtonPosition (left/right, defaults to ImGuiDir_Right) to move the color button
   of ColorEdit3/ColorEdit4 functions to either side of the inputs.

imgui.cpp

@@ -1042,6 +1042,7 @@ static const float NAV_WINDOWING_LIST_APPEAR_DELAY          = 0.15f;    // Time
 // Window resizing from edges (when io.ConfigWindowsResizeFromEdges = true and ImGuiBackendFlags_HasMouseCursors is set in io.BackendFlags by back-end)
 static const float WINDOWS_RESIZE_FROM_EDGES_HALF_THICKNESS = 4.0f;     // Extend outside and inside windows. Affect FindHoveredWindow().
 static const float WINDOWS_RESIZE_FROM_EDGES_FEEDBACK_TIMER = 0.04f;    // Reduce visual noise by only highlighting the border after a certain time.
+static const float WINDOWS_MOUSE_WHEEL_SCROLL_LOCK_TIMER    = 2.00f;    // Lock scrolled window (so it doesn't pick child windows that are scrolling through) for a certaint time, unless mouse moved.
 
 //-------------------------------------------------------------------------
 // [SECTION] FORWARD DECLARATIONS
@@ -3465,19 +3466,45 @@ static void ImGui::UpdateMouseInputs()
     }
 }
 
-void ImGui::UpdateMouseWheel()
+static void StartLockWheelingWindow(ImGuiWindow* window)
 {
     ImGuiContext& g = *GImGui;
-    if (!g.HoveredWindow || g.HoveredWindow->Collapsed)
+    if (g.WheelingWindow == window)
         return;
+    g.WheelingWindow = window;
+    g.WheelingWindowRefMousePos = g.IO.MousePos;
+    g.WheelingWindowTimer = WINDOWS_MOUSE_WHEEL_SCROLL_LOCK_TIMER;
+}
+
+void ImGui::UpdateMouseWheel()
+{
+    ImGuiContext& g = *GImGui;
+
+    // Reset the locked window if we move the mouse or after the timer elapses
+    if (g.WheelingWindow != NULL)
+    {
+        g.WheelingWindowTimer -= g.IO.DeltaTime;
+        if (IsMousePosValid() && ImLengthSqr(g.IO.MousePos - g.WheelingWindowRefMousePos) > g.IO.MouseDragThreshold * g.IO.MouseDragThreshold)
+            g.WheelingWindowTimer = 0.0f;
+        if (g.WheelingWindowTimer <= 0.0f)
+        {
+            g.WheelingWindow = NULL;
+            g.WheelingWindowTimer = 0.0f;
+        }
+    }
+
     if (g.IO.MouseWheel == 0.0f && g.IO.MouseWheelH == 0.0f)
         return;
 
+    ImGuiWindow* window = g.WheelingWindow ? g.WheelingWindow : g.HoveredWindow;
+    if (!window || window->Collapsed)
+        return;
+
     // Zoom / Scale window
     // FIXME-OBSOLETE: This is an old feature, it still works but pretty much nobody is using it and may be best redesigned.
     if (g.IO.MouseWheel != 0.0f && g.IO.KeyCtrl && g.IO.FontAllowUserScaling)
     {
-        ImGuiWindow* window = g.HoveredWindow;
+        StartLockWheelingWindow(window);
         const float new_font_scale = ImClamp(window->FontWindowScale + g.IO.MouseWheel * 0.10f, 0.50f, 2.50f);
         const float scale = new_font_scale / window->FontWindowScale;
         window->FontWindowScale = new_font_scale;
@@ -3493,13 +3520,12 @@ void ImGui::UpdateMouseWheel()
 
     // Mouse wheel scrolling
     // If a child window has the ImGuiWindowFlags_NoScrollWithMouse flag, we give a chance to scroll its parent
-    // FIXME: Lock scrolling window while not moving (see #2604)
 
     // Vertical Mouse Wheel scrolling
     const float wheel_y = (g.IO.MouseWheel != 0.0f && !g.IO.KeyShift) ? g.IO.MouseWheel : 0.0f;
     if (wheel_y != 0.0f && !g.IO.KeyCtrl)
     {
-        ImGuiWindow* window = g.HoveredWindow;
+        StartLockWheelingWindow(window);
         while ((window->Flags & ImGuiWindowFlags_ChildWindow) && ((window->ScrollMax.y == 0.0f) || ((window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(window->Flags & ImGuiWindowFlags_NoMouseInputs))))
             window = window->ParentWindow;
         if (!(window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(window->Flags & ImGuiWindowFlags_NoMouseInputs))
@@ -3514,7 +3540,7 @@ void ImGui::UpdateMouseWheel()
     const float wheel_x = (g.IO.MouseWheelH != 0.0f && !g.IO.KeyShift) ? g.IO.MouseWheelH : (g.IO.MouseWheel != 0.0f && g.IO.KeyShift) ? g.IO.MouseWheel : 0.0f;
     if (wheel_x != 0.0f && !g.IO.KeyCtrl)
     {
-        ImGuiWindow* window = g.HoveredWindow;
+        StartLockWheelingWindow(window);
         while ((window->Flags & ImGuiWindowFlags_ChildWindow) && ((window->ScrollMax.x == 0.0f) || ((window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(window->Flags & ImGuiWindowFlags_NoMouseInputs))))
             window = window->ParentWindow;
         if (!(window->Flags & ImGuiWindowFlags_NoScrollWithMouse) && !(window->Flags & ImGuiWindowFlags_NoMouseInputs))

imgui_internal.h

@@ -868,6 +868,9 @@ struct ImGuiContext
     ImGuiWindow*            HoveredWindow;                      // Will catch mouse inputs
     ImGuiWindow*            HoveredRootWindow;                  // Will catch mouse inputs (for focus/move only)
     ImGuiWindow*            MovingWindow;                       // Track the window we clicked on (in order to preserve focus). The actually window that is moved is generally MovingWindow->RootWindow.
+    ImGuiWindow*            WheelingWindow;
+    ImVec2                  WheelingWindowRefMousePos;
+    float                   WheelingWindowTimer;
 
     // Item/widgets state and tracking information
     ImGuiID                 HoveredId;                          // Hovered widget
@@ -1058,6 +1061,8 @@ struct ImGuiContext
         HoveredWindow = NULL;
         HoveredRootWindow = NULL;
         MovingWindow = NULL;
+        WheelingWindow = NULL;
+        WheelingWindowTimer = 0.0f;
 
         HoveredId = 0;
         HoveredIdAllowOverlap = false;