Viewports, Docking: Fixed a bug where closing a viewport using OS facilities would erroneously close all windows located in the viewport. (#8887)

docs/CHANGELOG.txt

@@ -70,6 +70,12 @@ Changes:
 - Backends: Allegro5: Fixed missing support for ImGuiKey_PrintScreen
   under Windows, as raw Allegro 5 does not receive it.
 
+Docking+Viewports Branch:
+
+- Fixed a bug where closing a viewport using OS facility (e.g. ALT+F4, Close Button)
+  would erroneously close all windows located in the viewport, even ones docked
+  into nested dockspaces. Only top-most windows should be closed. (#8887) [@lailoken]
+
 
 -----------------------------------------------------------------------
  VERSION 1.92.2 (Released 2025-08-11)

imgui.cpp

@@ -8373,12 +8373,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             NavInitWindow(window, false); // <-- this is in the way for us to be able to defer and sort reappearing FocusWindow() calls
 
         // Close requested by platform window (apply to all windows in this viewport)
+        // FIXME: Investigate removing the 'window->Viewport != GetMainViewport()' test, which seems superfluous.
         if (p_open != NULL && window->Viewport->PlatformRequestClose && window->Viewport != GetMainViewport())
-        {
-            IMGUI_DEBUG_LOG_VIEWPORT("[viewport] Window '%s' closed by PlatformRequestClose\n", window->Name);
-            *p_open = false;
-            g.NavWindowingToggleLayer = false; // Assume user mapped PlatformRequestClose on ALT-F4 so we disable ALT for menu toggle. False positive not an issue. // FIXME-NAV: Try removing.
-        }
+            if (window->DockNode == NULL || (window->DockNode->MergedFlags & ImGuiDockNodeFlags_DockSpace) == 0)
+            {
+                IMGUI_DEBUG_LOG_VIEWPORT("[viewport] Window '%s' closed by PlatformRequestClose\n", window->Name);
+                *p_open = false;
+                g.NavWindowingToggleLayer = false; // Assume user mapped PlatformRequestClose on ALT-F4 so we disable ALT for menu toggle. False positive not an issue. // FIXME-NAV: Try removing.
+            }
 
         // Pressing CTRL+C copy window content into the clipboard
         // [EXPERIMENTAL] Breaks on nested Begin/End pairs. We need to work that out and add better logging scope.