Viewports: update LastFocusedStampCount earlier. Fix (#8948, #9172, #9131, #9128) but probably have other side effects too.

Amend f1ae07e532. Initial value of LastFocusedStampCount was set at the time of platform ShowWindow which is too late for first-frame UpdateTryMergeWindowIntoHostViewport() evaluation.

docs/CHANGELOG.txt

@@ -210,9 +210,13 @@ Other Changes:
 
 Docking+Viewports Branch:
 
-- Fixed an assert in background dimming code, which could trigger after using
-  gamepad/keyboard to move a window to another viewport. (#9053) [@lut0pia, @ocornut]
 - Fixed implicit/fallback "Debug" window from staying visible if once docked. (#9151)
+- Viewports:
+  - Fixed a regression in 1.92.4 which could cause combos/popups window from
+    appearing under their parent viewport if their geometry overlapped the main
+    viewport. (#8948, #9172, #9131, #9128)
+  - Fixed an assert in background dimming code, which could trigger after using
+    gamepad/keyboard to move a window to another viewport. (#9053) [@lut0pia, @ocornut]
 - Backends:
   - GLFW: dynamically load X11 functions to avoid -lx11 linking requirement introduced
     in 1.92.3. (#9116, #9109) [@ramenguy99]

imgui.cpp

@@ -16989,6 +16989,10 @@ ImGuiViewportP* ImGui::AddUpdateViewport(ImGuiWindow* window, ImGuiID id, const
         g.ViewportCreatedCount++;
         IMGUI_DEBUG_LOG_VIEWPORT("[viewport] Add Viewport %08X '%s'\n", id, window ? window->Name : "<NULL>");
 
+        // We assume the window becomes front-most (even when ImGuiViewportFlags_NoFocusOnAppearing is used).
+        // This is useful for our platform z-order heuristic when io.MouseHoveredViewport is not available.
+        viewport->LastFocusedStampCount = ++g.ViewportFocusedStampCount;
+
         // We normally setup for all viewports in NewFrame() but here need to handle the mid-frame creation of a new viewport.
         // We need to extend the fullscreen clip rect so the OverlayDrawList clip is correct for that the first frame
         g.DrawListSharedData.ClipRectFullscreen.x = ImMin(g.DrawListSharedData.ClipRectFullscreen.x, viewport->Pos.x);
@@ -17363,11 +17367,6 @@ void ImGui::UpdatePlatformWindows()
 
             // Show window
             g.PlatformIO.Platform_ShowWindow(viewport);
-
-            // Even without focus, we assume the window becomes front-most.
-            // This is useful for our platform z-order heuristic when io.MouseHoveredViewport is not available.
-            if (viewport->LastFocusedStampCount != g.ViewportFocusedStampCount)
-                viewport->LastFocusedStampCount = ++g.ViewportFocusedStampCount;
         }
 
         // Clear request flags