Windows: fixed background order of overlapping childs submitted sequentially. (#4493)

Amend 07704496

docs/CHANGELOG.txt

@@ -42,6 +42,7 @@ Breaking Changes:
 
 Other Changes:
 
+- Windows: fixed background order of overlapping childs submitted sequentially. (#4493)
 - InputTextMultiline: Fixed label size not being included into window contents rect unless
   the whole widget is clipped.
 - TextUnformatted: Accept null ranges including (NULL,NULL) without asserting, in order to conform

imgui.cpp

@@ -6214,16 +6214,21 @@ 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.
+        // Child windows can render their decoration (bg color, border, scrollbars, etc.) within their parent to save a draw call (since 1.71)
         // 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.
+        // FIXME: User 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 (github #4493)
         {
             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)
+            {
+                // - We test overlap with the previous child window only (testing all would end up being O(log N) not a good investment here)
+                // - We disable this when the parent window has zero vertices, which is a common pattern leading to laying out multiple overlapping childs
+                ImGuiWindow* previous_child = parent_window->DC.ChildWindows.Size >= 2 ? parent_window->DC.ChildWindows[parent_window->DC.ChildWindows.Size - 2] : NULL;
+                bool previous_child_overlapping = previous_child ? previous_child->Rect().Overlaps(window->Rect()) : false;
+                bool parent_is_empty = parent_window->DrawList->VtxBuffer.Size > 0;
+                if (window->DrawList->CmdBuffer.back().ElemCount == 0 && parent_is_empty && !previous_child_overlapping)
                     render_decorations_in_parent = true;
+            }
             if (render_decorations_in_parent)
                 window->DrawList = parent_window->DrawList;
 

imgui.h

@@ -61,7 +61,7 @@ Index of this file:
 // Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals. Work in progress versions typically starts at XYY99 then bounce up to XYY00, XYY01 etc. when release tagging happens)
 #define IMGUI_VERSION               "1.85 WIP"
-#define IMGUI_VERSION_NUM           18408
+#define IMGUI_VERSION_NUM           18409
 #define IMGUI_CHECKVERSION()        ImGui::DebugCheckVersionAndDataLayout(IMGUI_VERSION, sizeof(ImGuiIO), sizeof(ImGuiStyle), sizeof(ImVec2), sizeof(ImVec4), sizeof(ImDrawVert), sizeof(ImDrawIdx))
 #define IMGUI_HAS_TABLE