Menus: Fixed an assertion happening in some situations when closing nested menus (broken in 1.83). (#4640)

Broken since 936f5322
Weirdly chain-reaction caused by the fact following #4640 repro, the SourceWindow assignment in OpenPopupEx() picks Menu_04 before its closure. Value of SourceWindow since bda2cde6 #2517

docs/CHANGELOG.txt

@@ -86,9 +86,11 @@ Other Changes:
   RGB<>HSV round trips every frames. [@rokups]
 - ColorPicker4: Fixed picker being unable to select exact 1.0f color when dragging toward the edges
   of the SV square (previously picked 0.999989986f). (#3517) [@rokups]
-- Menus: Fixed vertical alignments of MenuItem() calls within a menu bar. (broken in 1.84). (#4538)
+- Menus: Fixed vertical alignments of MenuItem() calls within a menu bar (broken in 1.84). (#4538)
-- Menus: Adjust closing logic to accomodate for varying font size and dpi.
+- Menus: Improve closing logic when moving diagonally in empty between between parent and child menus to
+  accomodate for varying font size and dpi.
 - Menus: Fixed crash when navigating left inside a child window inside a sub-menu. (#4510).
+- Menus: Fixed an assertion happening in some situations when closing nested menus (broken in 1.83). (#4640)
 - Drag and Drop: Fixed using BeginDragDropSource() inside a BeginChild() that returned false. (#4515)
 - PlotHistogram: Fixed zero-line position when manually specifying min<0 and max>0. (#4349) [@filippocrocchini]
 - Misc: Added asserts for missing PopItemFlag() calls.

imgui.cpp

@@ -6551,8 +6551,18 @@ void ImGui::FocusWindow(ImGuiWindow* window)
 void ImGui::FocusTopMostWindowUnderOne(ImGuiWindow* under_this_window, ImGuiWindow* ignore_window)
 {
     ImGuiContext& g = *GImGui;
-
+    int start_idx = g.WindowsFocusOrder.Size - 1;
-    const int start_idx = ((under_this_window != NULL) ? FindWindowFocusIndex(under_this_window) : g.WindowsFocusOrder.Size) - 1;
+    if (under_this_window != NULL)
+    {
+        // Aim at root window behind us, if we are in a child window that's our own root (see #4640)
+        int offset = -1;
+        while (under_this_window->Flags & ImGuiWindowFlags_ChildWindow)
+        {
+            under_this_window = under_this_window->ParentWindow;
+            offset = 0;
+        }
+        start_idx = FindWindowFocusIndex(under_this_window) + offset;
+    }
     for (int i = start_idx; i >= 0; i--)
     {
         // We may later decide to test for different NoXXXInputs based on the active navigation input (mouse vs nav) but that may feel more confusing to the user.
@@ -9838,6 +9848,7 @@ static int ImGui::FindWindowFocusIndex(ImGuiWindow* window)
     ImGuiContext& g = *GImGui;
     IM_UNUSED(g);
     int order = window->FocusOrder;
+    IM_ASSERT((window->Flags & ImGuiWindowFlags_ChildWindow) == 0);
     IM_ASSERT(g.WindowsFocusOrder[order] == window);
     return order;
 }