Menus: reuse more menu windows when stacked over popups.

This is actually mostly aimed at ensuring the _ChildWindow flag on menu window doesn't vary as this would cause issues with our current g.WindowsFocusOrder[] scheme

imgui.cpp

@@ -5990,6 +5990,8 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     window_stack_data.StackSizesOnBegin.SetToCurrentState();
     g.CurrentWindowStack.push_back(window_stack_data);
     g.CurrentWindow = NULL;
+    if (flags & ImGuiWindowFlags_ChildMenu)
+        g.BeginMenuCount++;
 
     if (flags & ImGuiWindowFlags_Popup)
     {
@@ -6595,6 +6597,8 @@ void ImGui::End()
 
     // Pop from window stack
     g.LastItemData = g.CurrentWindowStack.back().ParentLastItemDataBackup;
+    if (window->Flags & ImGuiWindowFlags_ChildMenu)
+        g.BeginMenuCount--;
     if (window->Flags & ImGuiWindowFlags_Popup)
         g.BeginPopupStack.pop_back();
     g.CurrentWindowStack.back().StackSizesOnBegin.CompareWithCurrentState();
@@ -8710,7 +8714,7 @@ bool ImGui::BeginPopupEx(ImGuiID id, ImGuiWindowFlags flags)
 
     char name[20];
     if (flags & ImGuiWindowFlags_ChildMenu)
-        ImFormatString(name, IM_ARRAYSIZE(name), "##Menu_%02d", g.BeginPopupStack.Size); // Recycle windows based on depth
+        ImFormatString(name, IM_ARRAYSIZE(name), "##Menu_%02d", g.BeginMenuCount); // Recycle windows based on depth
     else
         ImFormatString(name, IM_ARRAYSIZE(name), "##Popup_%08x", id); // Not recycling, so we can close/open during the same frame
 

imgui_internal.h

@@ -1580,6 +1580,7 @@ struct ImGuiContext
     ImVector<ImGuiGroupData>GroupStack;                         // Stack for BeginGroup()/EndGroup() - not inherited by Begin()
     ImVector<ImGuiPopupData>OpenPopupStack;                     // Which popups are open (persistent)
     ImVector<ImGuiPopupData>BeginPopupStack;                    // Which level of BeginPopup() we are in (reset every frame)
+    int                     BeginMenuCount;
 
     // Viewports
     ImVector<ImGuiViewportP*> Viewports;                        // Active viewports (Size==1 in 'master' branch). Each viewports hold their copy of ImDrawData.
@@ -1801,6 +1802,7 @@ struct ImGuiContext
         LastActiveIdTimer = 0.0f;
 
         CurrentItemFlags = ImGuiItemFlags_None;
+        BeginMenuCount = 0;
 
         NavWindow = NULL;
         NavId = NavFocusScopeId = NavActivateId = NavActivateDownId = NavActivatePressedId = NavActivateInputId = 0;