Viewport: Fixed a viewport bug which led some popups to merge in underlying host viewport when they should be z-over their parent viewport.

docs/TODO.txt

@@ -287,7 +287,7 @@ It's mostly a bunch of personal notes, probably incomplete. Feel free to query i
  - viewport: with platform decoration enabled, platform may force constraint (e.g. minimum size)
  - viewport: use getfocus/setfocus api to synchronize imgui<>platform focus better (e.g imgui-side ctrl-tab can focus os window, OS initial setup and alt-tab can focus imgui window etc.)
  - viewport: store per-viewport/monitor DPI in .ini file so an application reload or main window changing DPI on reload can be properly patched for.
- - viewport: implicit Debug window can hog a zombie viewport (harmless, noisy?) > could at least clear out the reference on a per session basis?
+ - viewport: implicit/fallback Debug window can hog a zombie viewport (harmless, noisy?) > could at least clear out the reference on a per session basis?
  - viewport: need to clarify how to use GetMousePos() from a user point of view.
  - platform: glfw: no support for ImGuiBackendFlags_HasMouseHoveredViewport. 
  - platform: sdl: no support for ImGuiBackendFlags_HasMouseHoveredViewport. maybe we could use SDL_GetMouseFocus() / SDL_WINDOW_MOUSE_FOCUS if imgui could fallback on its heuristic when NoInputs is set

imgui.cpp

@@ -7487,10 +7487,6 @@ static void ImGui::UpdateSelectWindowViewport(ImGuiWindow* window)
         return;
     }
 
-    // Merge into host viewport
-    bool try_to_merge_into_host_viewport = false;
-    if (window->ViewportOwned && g.ActiveId == 0)
-        try_to_merge_into_host_viewport = true;
     window->ViewportOwned = false;
 
     // Appearing popups reset their viewport so they can inherit again
@@ -7541,6 +7537,9 @@ static void ImGui::UpdateSelectWindowViewport(ImGuiWindow* window)
     }
     else
     {
+        // Merge into host viewport?
+        // We cannot test window->ViewportOwned as it set lower in the function.
+        bool try_to_merge_into_host_viewport = (window->Viewport && window == window->Viewport->Window && g.ActiveId == 0);
         if (try_to_merge_into_host_viewport)
             UpdateTryMergeWindowIntoHostViewport(window, g.Viewports[0]);
     }
@@ -7594,7 +7593,7 @@ void ImGui::UpdatePlatformWindows()
     {
         ImGuiViewportP* viewport = g.Viewports[i];
 
-        // Destroy platform window if the viewport hasn't been submitted or if it is hosting a hidden window (the implicit Debug window will be registered its viewport then be disabled)
+        // Destroy platform window if the viewport hasn't been submitted or if it is hosting a hidden window (the implicit/fallback Debug window will be registered its viewport then be disabled)
         bool destroy_platform_window = false;
         destroy_platform_window |= (viewport->LastFrameActive < g.FrameCount - 1);
         destroy_platform_window |= (viewport->Window && !IsWindowActiveAndVisible(viewport->Window));