Viewports: Fixed loss of imgui-side focus when dragging a secondary viewport back in main viewport. (#6299)

Amend d2291df55, 63370be0e, dcb6335bfe

docs/CHANGELOG.txt

@@ -153,6 +153,8 @@ Docking+Viewports Branch:
 
 - Viewports: Fixed platform-side focus (e.g. Alt+Tab) from leading to accidental
   closure of Modal windows. Regression from 1.89.5. (#6357, #6299)
+- Viewports: Fixed loss of imgui-side focus when dragging a secondary viewport back in
+  main viewport, due to platform-side handling changes. Regression from 1.89.5 (#6299)
 - Viewports: Added void* ImGuiPlatformMonitor::PlatformHandle field (backend-dependant),
   for usage by user code.
 - Backends: SDL2: Update monitor list when receiving a display event. (#6348)

imgui.cpp

@@ -14000,6 +14000,7 @@ static void ImGui::UpdateViewportsNewFrame()
             // FIXME: perhaps 'FocusTopMostWindowUnderOne()' can handle the 'focused_window->Window != NULL' case as well.
             if (!IsAnyMouseDown())
             {
+                focused_viewport->LastFocusedHadNavWindow |= (g.NavWindow != NULL) && (g.NavWindow->Viewport == focused_viewport); // Update so a window changing viewport won't lose focus.
                 ImGuiFocusRequestFlags focus_request_flags = ImGuiFocusRequestFlags_UnlessBelowModal | ImGuiFocusRequestFlags_RestoreFocusedChild;
                 if (focused_viewport->Window != NULL)
                     FocusWindow(focused_viewport->Window, focus_request_flags);