Docking: Fix crash when a dock node gets re-qualified as dockspace>floating>dockspace.. (#3203, #4295)

Which tends to happen when incorrectly calling DockBuilderAddNode() without ImGuiDockNodeFlags_Dockspace and using it as a Dockspace on the next frame after the floating window hosting the node has been automatically created.

docs/CHANGELOG.txt

@@ -144,7 +144,10 @@ Docking+Viewports Branch:
 
 - Docking: Clicking on the right-most close button of a docking node closes all windows. (#4186)
 - Docking: Fix IsWindowAppearing() and ImGuiCond_Appearing on docked windows. (#4177, #3982, #1497, #1061)
-- Docking: Fix crash issues using DockBuilderRemoveNode() in some situations. (#3111, #3179, #3203, #4295) [@hsimyu]
+- Docking: Fix crash using DockBuilderRemoveNode() in some situations. (#3111, #3179, #3203, #4295) [@hsimyu]
+- Docking: Fix crash when a dock node gets re-qualified as dockspace>floating>dockspace, which tends to happen
+  when incorrectly calling DockBuilderAddNode() without ImGuiDockNodeFlags_Dockspace and using it as a Dockspace
+  on the next frame after the floating window hosting the node has been automatically created. (#3203, #4295)
 - Docking: Reworked node flags saving/inheritance so that flags enforced by docked windows via the
   DockNodeFlagsOverrideSet mechanism are are not left in empty dockspace nodes once the windows gets undocked.
   (#4292, #3834, #3633, #3521, #3492, #3335, #2999, #2648)

imgui.cpp

@@ -13520,6 +13520,20 @@ static void ImGui::DockNodeUpdateForRootNode(ImGuiDockNode* node)
     }
 }
 
+static void DockNodeSetupHostWindow(ImGuiDockNode* node, ImGuiWindow* host_window)
+{
+    // Remove ourselves from any previous different host window
+    // This can happen if a user mistakenly does (see #4295 for details):
+    //  - N+0: DockBuilderAddNode(id, 0)    // missing ImGuiDockNodeFlags_DockSpace
+    //  - N+1: NewFrame()                   // will create floating host window for that node
+    //  - N+1: DockSpace(id)                // requalify node as dockspace, moving host window
+    if (node->HostWindow && node->HostWindow != host_window && node->HostWindow->DockNodeAsHost == node)
+        node->HostWindow->DockNodeAsHost = NULL;
+
+    host_window->DockNodeAsHost = node;
+    node->HostWindow = host_window;
+}
+
 static void ImGui::DockNodeUpdate(ImGuiDockNode* node)
 {
     ImGuiContext& g = *GImGui;
@@ -13668,8 +13682,8 @@ static void ImGui::DockNodeUpdate(ImGuiDockNode* node)
             PopStyleVar();
             beginned_into_host_window = true;
 
-            node->HostWindow = host_window = g.CurrentWindow;
-            host_window->DockNodeAsHost = node;
+            host_window = g.CurrentWindow;
+            DockNodeSetupHostWindow(node, host_window);
             host_window->DC.CursorPos = host_window->Pos;
             node->Pos = host_window->Pos;
             node->Size = host_window->Size;
@@ -14986,9 +15000,8 @@ ImGuiID ImGui::DockSpace(ImGuiID id, const ImVec2& size_arg, ImGuiDockNodeFlags
     PopStyleVar();
 
     ImGuiWindow* host_window = g.CurrentWindow;
-    host_window->DockNodeAsHost = node;
+    DockNodeSetupHostWindow(node, host_window);
     host_window->ChildId = window->GetID(title);
-    node->HostWindow = host_window;
     node->OnlyNodeWithWindows = NULL;
 
     IM_ASSERT(node->IsRootNode());