Internals: Minor no-op tidying up toward solving the WindowPadding / WindowBorderSize / ScrollbarSize overlapping mess.

+ Demo: Use SetScrollY().

imgui.cpp

@@ -5400,6 +5400,9 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->SizeFull = CalcSizeAfterConstraint(window, window->SizeFull);
         window->Size = window->Collapsed && !(flags & ImGuiWindowFlags_ChildWindow) ? window->TitleBarRect().GetSize() : window->SizeFull;
 
+        // Decoration size
+        const float decoration_up_height = window->TitleBarHeight() + window->MenuBarHeight();
+
         // SCROLLBAR STATUS
 
         // Update scrollbar status (based on the Size that was effective during last frame or the auto-resized Size).
@@ -5407,7 +5410,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         {
             // When reading the current size we need to read it after size constraints have been applied.
             // When we use InnerRect here we are intentionally reading last frame size, same for ScrollbarSizes values before we set them again.
-            ImVec2 avail_size_from_current_frame = ImVec2(window->SizeFull.x, window->SizeFull.y - window->TitleBarHeight() - window->MenuBarHeight());
+            ImVec2 avail_size_from_current_frame = ImVec2(window->SizeFull.x, window->SizeFull.y - decoration_up_height);
             ImVec2 avail_size_from_last_frame = window->InnerRect.GetSize() + window->ScrollbarSizes;
             ImVec2 needed_size_from_last_frame = window_just_created ? ImVec2(0,0) : window->ContentSize + window->WindowPadding * 2.0f;
             float size_x_for_scrollbars = (size_full_modified.x != FLT_MAX || window_just_created) ? avail_size_from_current_frame.x : avail_size_from_last_frame.x;
@@ -5499,19 +5502,20 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // - FindHoveredWindow() (w/ extra padding when border resize is enabled)
         // - Begin() initial clipping rect for drawing window background and borders.
         // - Begin() clipping whole child
-        ImRect host_rect = ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup) && !window_is_child_tooltip) ? parent_window->ClipRect : viewport_rect;
-        ImRect outer_rect = window->Rect();
+        const ImRect host_rect = ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup) && !window_is_child_tooltip) ? parent_window->ClipRect : viewport_rect;
+        const ImRect outer_rect = window->Rect();
+        const ImRect title_bar_rect = window->TitleBarRect();
         window->OuterRectClipped = outer_rect;
         window->OuterRectClipped.ClipWith(host_rect);
 
         // Inner rectangle
         // Not affected by window border size. Used by: 
+        // - InnerClipRect
         // - NavScrollToBringItemIntoView()
         // - NavUpdatePageUpPageDown()
         // - Scrollbar()
-        const ImRect title_bar_rect = window->TitleBarRect();
-        window->InnerRect.Min.x = title_bar_rect.Min.x;
-        window->InnerRect.Min.y = title_bar_rect.Max.y + window->MenuBarHeight();
+        window->InnerRect.Min.x = window->Pos.x;
+        window->InnerRect.Min.y = window->Pos.y + decoration_up_height;
         window->InnerRect.Max.x = window->Pos.x + window->Size.x - window->ScrollbarSizes.x;
         window->InnerRect.Max.y = window->Pos.y + window->Size.y - window->ScrollbarSizes.y;
 
@@ -5606,7 +5610,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         // Used by:
         // - Mouse wheel scrolling + many other things
         window->ContentsRegionRect.Min.x = window->Pos.x - window->Scroll.x + window->WindowPadding.x;
-        window->ContentsRegionRect.Min.y = window->Pos.y - window->Scroll.y + window->WindowPadding.y + window->TitleBarHeight() + window->MenuBarHeight();
+        window->ContentsRegionRect.Min.y = window->Pos.y - window->Scroll.y + window->WindowPadding.y + decoration_up_height;
         window->ContentsRegionRect.Max.x = window->Pos.x - window->Scroll.x - window->WindowPadding.x + (window->ContentSizeExplicit.x != 0.0f ? window->ContentSizeExplicit.x : (window->Size.x - window->ScrollbarSizes.x + ImMin(window->ScrollbarSizes.x, window->WindowBorderSize)));
         window->ContentsRegionRect.Max.y = window->Pos.y - window->Scroll.y - window->WindowPadding.y + (window->ContentSizeExplicit.y != 0.0f ? window->ContentSizeExplicit.y : (window->Size.y - window->ScrollbarSizes.y + ImMin(window->ScrollbarSizes.y, window->WindowBorderSize)));
 
@@ -5615,7 +5619,7 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         window->DC.Indent.x = 0.0f + window->WindowPadding.x - window->Scroll.x;
         window->DC.GroupOffset.x = 0.0f;
         window->DC.ColumnsOffset.x = 0.0f;
-        window->DC.CursorStartPos = window->Pos + ImVec2(window->DC.Indent.x + window->DC.ColumnsOffset.x, window->TitleBarHeight() + window->MenuBarHeight() + window->WindowPadding.y - window->Scroll.y);
+        window->DC.CursorStartPos = window->Pos + ImVec2(window->DC.Indent.x + window->DC.ColumnsOffset.x, decoration_up_height + window->WindowPadding.y - window->Scroll.y);
         window->DC.CursorPos = window->DC.CursorStartPos;
         window->DC.CursorPosPrevLine = window->DC.CursorPos;
         window->DC.CursorMaxPos = window->DC.CursorStartPos;

imgui_demo.cpp

@@ -2059,14 +2059,21 @@ static void ShowDemoWindowLayout()
         HelpMarker("Use SetScrollHereY() or SetScrollFromPosY() to scroll to a given position.");
 
         static bool track = true;
-        static int track_line = 50, scroll_to_px = 200;
+        static int track_line = 50;
+        static float scroll_to_off_px = 0.0f;
+        static float scroll_to_pos_px = 200.0f;
         ImGui::Checkbox("Track", &track);
         ImGui::PushItemWidth(100);
-        ImGui::SameLine(130); track |= ImGui::DragInt("##line", &track_line, 0.25f, 0, 99, "Line = %d");
-        bool scroll_to = ImGui::Button("Scroll To Pos");
-        ImGui::SameLine(130); scroll_to |= ImGui::DragInt("##pos_y", &scroll_to_px, 1.00f, 0, 9999, "Y = %d px");
+        ImGui::SameLine(140); track |= ImGui::DragInt("##line", &track_line, 0.25f, 0, 99, "Line = %d");
+
+        bool scroll_to_off = ImGui::Button("Scroll Offset");
+        ImGui::SameLine(140); scroll_to_off |= ImGui::DragFloat("##off_y", &scroll_to_off_px, 1.00f, 0, 9999, "+%.0f px");
+
+        bool scroll_to_pos = ImGui::Button("Scroll To Pos");
+        ImGui::SameLine(140); scroll_to_pos |= ImGui::DragFloat("##pos_y", &scroll_to_pos_px, 1.00f, 0, 9999, "Y = %.0f px");
+
         ImGui::PopItemWidth();
-        if (scroll_to) 
+        if (scroll_to_off || scroll_to_pos)
             track = false;
 
         ImGuiStyle& style = ImGui::GetStyle();
@@ -2076,9 +2083,13 @@ static void ShowDemoWindowLayout()
             if (i > 0) ImGui::SameLine();
             ImGui::BeginGroup();
             ImGui::Text("%s", i == 0 ? "Top" : i == 1 ? "25%" : i == 2 ? "Center" : i == 3 ? "75%" : "Bottom");
-            ImGui::BeginChild(ImGui::GetID((void*)(intptr_t)i), ImVec2(child_w, 200.0f), true);
-            if (scroll_to)
-                ImGui::SetScrollFromPosY(ImGui::GetCursorStartPos().y + scroll_to_px, i * 0.25f);
+
+            ImGuiWindowFlags child_flags = ImGuiWindowFlags_MenuBar;
+            ImGui::BeginChild(ImGui::GetID((void*)(intptr_t)i), ImVec2(child_w, 200.0f), true, child_flags);
+            if (scroll_to_off)
+                ImGui::SetScrollY(scroll_to_off_px);
+            if (scroll_to_pos)
+                ImGui::SetScrollFromPosY(ImGui::GetCursorStartPos().y + scroll_to_pos_px, i * 0.25f);
             for (int line = 0; line < 100; line++)
             {
                 if (track && line == track_line)
@@ -2094,7 +2105,7 @@ static void ShowDemoWindowLayout()
             float scroll_y = ImGui::GetScrollY();
             float scroll_max_y = ImGui::GetScrollMaxY();
             ImGui::EndChild();
-            ImGui::Text("%.0f/%0.f", scroll_y, scroll_max_y);
+            ImGui::Text("%.0f/%.0f", scroll_y, scroll_max_y);
             ImGui::EndGroup();
         }
         ImGui::TreePop();

imgui_widgets.cpp

@@ -883,22 +883,25 @@ void ImGui::Scrollbar(ImGuiAxis axis)
 
     // Calculate scrollbar bounding box
     const ImRect outer_rect = window->Rect();
+    const ImRect inner_rect = window->InnerRect;
+    const float scrollbar_size = window->ScrollbarSizes[axis ^ 1];
+    IM_ASSERT(scrollbar_size > 0.0f); IM_UNUSED(scrollbar_size);
     const float other_scrollbar_size = window->ScrollbarSizes[axis];
     ImDrawCornerFlags rounding_corners = (other_scrollbar_size <= 0.0f) ? ImDrawCornerFlags_BotRight : 0;
     ImRect bb;
     if (axis == ImGuiAxis_X)
     {
-        bb.Min = ImVec2(window->InnerRect.Min.x, window->InnerRect.Max.y);
-        bb.Max = ImVec2(window->InnerRect.Max.x, outer_rect.Max.y - window->WindowBorderSize);
+        bb.Min = ImVec2(inner_rect.Min.x, inner_rect.Max.y);
+        bb.Max = ImVec2(inner_rect.Max.x, outer_rect.Max.y - window->WindowBorderSize);
         rounding_corners |= ImDrawCornerFlags_BotLeft;
     }
     else
     {
-        bb.Min = ImVec2(window->InnerRect.Max.x, window->InnerRect.Min.y);
+        bb.Min = ImVec2(inner_rect.Max.x, inner_rect.Min.y);
         bb.Max = ImVec2(outer_rect.Max.x - window->WindowBorderSize, window->InnerRect.Max.y);
         rounding_corners |= ((window->Flags & ImGuiWindowFlags_NoTitleBar) && !(window->Flags & ImGuiWindowFlags_MenuBar)) ? ImDrawCornerFlags_TopRight : 0;
     }
-    ScrollbarEx(bb, id, axis, &window->Scroll[axis], window->InnerRect.Max[axis] - window->InnerRect.Min[axis], window->ContentSize[axis] + window->WindowPadding[axis] * 2.0f, rounding_corners);
+    ScrollbarEx(bb, id, axis, &window->Scroll[axis], inner_rect.Max[axis] - inner_rect.Min[axis], window->ContentSize[axis] + window->WindowPadding[axis] * 2.0f, rounding_corners);
 }
 
 void ImGui::Image(ImTextureID user_texture_id, const ImVec2& size, const ImVec2& uv0, const ImVec2& uv1, const ImVec4& tint_col, const ImVec4& border_col)