Window: child windows outer decorations (e.g. scrollbar) are rendered as part of their parent window, avoiding the creation of an extraneous draw command.

+ Metrics: inverted color of clip rect vs vertices bounding box when hovering a draw command, so the color matches the per-vertex preview.

docs/CHANGELOG.txt

@@ -35,6 +35,12 @@ HOW TO UPDATE?
 Breaking Changes:
 - IO: changed AddInputCharacter(unsigned short c) signature to AddInputCharacter(unsigned int c).
 - Renamed SetNextTreeNodeOpen() to SetNextItemOpen(). Kept inline redirection function (will obsolete).
+- Window: rendering of child windows outer decorations (e.g. bg color, border, scrollbars) is now
+  performed as part of their parent window, avoiding the creation of an extraneous draw commands.
+  If you have overlapping child windows with decorations, and relied on their relative z-order to be
+  mapped to submission their order, this will affect your rendering. The optimization is disabled
+  if the parent window has no visual output because it appears to be the most common situation leading
+  to the creation of overlapping child windows. Please reach out if you are affected by this change!
 
 Other Changes:
 - Window: clarified behavior of SetNextWindowContentSize(). Content size is defined as the size available
@@ -44,8 +50,6 @@ Other Changes:
 - Window: Fixed auto-resize with AlwaysVerticalScrollbar or AlwaysHorizontalScrollbar flags.
 - Window: Fixed one case where auto-resize by double-clicking the resize grip would make either scrollbar
   appear for a single frame after the resize.
-- Columns: Fixed Separator from creating an extraneous draw command. (#125)
-- Columns: Fixed Selectable with SpanAllColumns flag from creating an extraneous draw command. (#125)
 - Separator: Revert 1.70 "Declare its thickness (1.0f) to the layout" change. It's not incorrect
   but it breaks existing some layout patterns. Will return back to it when we expose Separator flags.
 - Fixed InputScalar, InputScalarN, SliderScalarN, DragScalarN with non-visible label from inserting
@@ -74,6 +78,8 @@ Other Changes:
   loop with the horizontal contents size.
 - Columns: Fixed Columns() within a window with horizontal scrolling from not covering the full
   horizontal area (previously only worked with an explicit contents size). (#125)
+- Columns: Fixed Separator from creating an extraneous draw command. (#125)
+- Columns: Fixed Selectable with SpanAllColumns flag from creating an extraneous draw command. (#125)
 - Style: Added style.WindowMenuButtonPosition (left/right, defaults to ImGuiDir_Left) to move the
   collapsing/docking button to the other side of the title bar.
 - Style: Made window close button cross slightly smaller.

imgui.cpp

@@ -369,6 +369,10 @@ CODE
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2019/06/07 (1.71) - rendering of child window outer decorations (bg color, border, scrollbars) is now performed as part of the parent window. If you have
+                       overlapping child windows in a same parent, and relied on their relative z-order to be mapped to their submission order, this will affect your rendering.
+                       This optimization is disabled if the parent window has no visual output, because it appears to be the most common situation leading to the creation of overlapping child windows.
+                       Please reach out if you are affected.
  - 2019/05/13 (1.71) - renamed SetNextTreeNodeOpen() to SetNextItemOpen(). Kept inline redirection function (will obsolete).
  - 2019/05/11 (1.71) - changed io.AddInputCharacter(unsigned short c) signature to io.AddInputCharacter(unsigned int c).
  - 2019/04/29 (1.70) - improved ImDrawList thick strokes (>1.0f) preserving correct thickness up to 90 degrees angles (e.g. rectangles). If you have custom rendering using thick lines, they will appear thicker now.
@@ -3768,7 +3772,7 @@ static void AddWindowToSortBuffer(ImVector<ImGuiWindow*>* out_sorted_windows, Im
     {
         int count = window->DC.ChildWindows.Size;
         if (count > 1)
-            ImQsort(window->DC.ChildWindows.begin(), (size_t)count, sizeof(ImGuiWindow*), ChildWindowComparer);
+            ImQsort(window->DC.ChildWindows.Data, (size_t)count, sizeof(ImGuiWindow*), ChildWindowComparer);
         for (int i = 0; i < count; i++)
         {
             ImGuiWindow* child = window->DC.ChildWindows[i];
@@ -5603,10 +5607,25 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
                 window->DrawList->AddRectFilled(bb.Min, bb.Max, GetColorU32(ImGuiCol_NavWindowingHighlight, g.NavWindowingHighlightAlpha * 0.25f), g.Style.WindowRounding);
         }
 
+        // Since 1.71, child window can render their decoration (bg color, border, scrollbars, etc.) within their parent to save a draw call.
+        // When using overlapping child windows, this will break the assumption that child z-order is mapped to submission order.
+        // We disable this when the parent window has zero vertices, which is a common pattern leading to laying out multiple overlapping child.
+        // We also disabled this when we have dimming overlay behind this specific one child.
+        // FIXME: More code may rely on explicit sorting of overlapping child window and would need to disable this somehow. Please get in contact if you are affected.
+        bool render_decorations_in_parent = false;
+        if ((flags & ImGuiWindowFlags_ChildWindow) && !(flags & ImGuiWindowFlags_Popup) && !window_is_child_tooltip)
+            if (window->DrawList->CmdBuffer.back().ElemCount == 0 && parent_window->DrawList->VtxBuffer.Size > 0)
+                render_decorations_in_parent = true;
+        if (render_decorations_in_parent)
+            window->DrawList = parent_window->DrawList;
+        
         const ImGuiWindow* window_to_highlight = g.NavWindowingTarget ? g.NavWindowingTarget : g.NavWindow;
         const bool title_bar_is_highlight = want_focus || (window_to_highlight && window->RootWindowForTitleBarHighlight == window_to_highlight->RootWindowForTitleBarHighlight);
         RenderWindowDecorations(window, title_bar_rect, title_bar_is_highlight, resize_grip_count, resize_grip_col, resize_grip_draw_size);
 
+        if (render_decorations_in_parent)
+            window->DrawList = &window->DrawListInst;
+
         // Draw navigation selection/windowing rectangle border
         if (g.NavWindowingTargetAnim == window)
         {
@@ -9883,8 +9902,8 @@ void ImGui::ShowMetricsWindow(bool* p_open)
                     ImRect vtxs_rect;
                     for (int i = elem_offset; i < elem_offset + (int)pcmd->ElemCount; i++)
                         vtxs_rect.Add(draw_list->VtxBuffer[idx_buffer ? idx_buffer[i] : i].pos);
-                    clip_rect.Floor(); fg_draw_list->AddRect(clip_rect.Min, clip_rect.Max, IM_COL32(255,255,0,255));
+                    clip_rect.Floor(); fg_draw_list->AddRect(clip_rect.Min, clip_rect.Max, IM_COL32(255,0,255,255));
-                    vtxs_rect.Floor(); fg_draw_list->AddRect(vtxs_rect.Min, vtxs_rect.Max, IM_COL32(255,0,255,255));
+                    vtxs_rect.Floor(); fg_draw_list->AddRect(vtxs_rect.Min, vtxs_rect.Max, IM_COL32(255,255,0,255));
                 }
                 if (!pcmd_node_open)
                     continue;