Fixed implicit/fallback "Debug" window from staying visible if once docked. (#9151)

docs/CHANGELOG.txt

diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 3029785..3f6644d 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -156,6 +156,7 @@
 
 - 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)
 - Backends:
   - Win32: viewports created by backend forcefully direct messages to
     DefWindowProcW() in order to support Unicode text input. (#9099, #3653, #5961) [@ulhc]

imgui.cpp

diff --git a/imgui.cpp b/imgui.cpp
index 5bdfbee..dba0246 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -6201,7 +6201,7 @@
 
     // Hide implicit/fallback "Debug" window if it hasn't been used
     g.WithinFrameScopeWithImplicitWindow = false;
-    if (g.CurrentWindow && !g.CurrentWindow->WriteAccessed)
+    if (g.CurrentWindow && g.CurrentWindow->IsFallbackWindow && g.CurrentWindow->WriteAccessed == false)
         g.CurrentWindow->Active = false;
     End();
 
@@ -18698,7 +18698,7 @@
 
         bool node_was_active = (node->LastFrameActive + 1 == g.FrameCount);
         bool remove = false;
-        remove |= node_was_active && (window->LastFrameActive + 1 < g.FrameCount);
+        remove |= node_was_active && (window->WasActive == false); // Can't use 'window->LastFrameActive + 1 < g.FrameCount'. (see #9151)
         remove |= node_was_active && (node->WantCloseAll || node->WantCloseTabId == window->TabId) && window->HasCloseButton && !(window->Flags & ImGuiWindowFlags_UnsavedDocument);  // Submit all _expected_ closure from last frame
         remove |= (window->DockTabWantClose);
         if (remove)
@@ -19405,7 +19405,7 @@
     {
         ImGuiWindow* window = node->Windows[window_n];
         if (window->LastFrameActive + 1 < g.FrameCount && node_was_active)
-            continue; // FIXME: Not sure if that's still taken/useful.
+            continue; // FIXME: Not sure if that's still taken/useful, as windows are normally removed in DockNodeUpdateFlagsAndCollapse().
 
         ImGuiTabItemFlags tab_item_flags = 0;
         tab_item_flags |= window->WindowClass.TabItemFlagsOverrideSet;
@@ -20994,6 +20994,13 @@
     // Clear fields ahead so most early-out paths don't have to do it
     window->DockIsActive = window->DockNodeIsVisible = window->DockTabIsVisible = false;
 
+    // Specific extra processing for fallback window (#9151), could be in Begin() as well.
+    if (window->IsFallbackWindow && !window->WasActive)
+    {
+        DockNodeHideWindowDuringHostWindowCreation(window);
+        return;
+    }
+
     const bool auto_dock_node = GetWindowAlwaysWantOwnTabBar(window);
     if (auto_dock_node)
     {