Docking: prevent docking any window created above a popup/modal. (#4317)

docs/CHANGELOG.txt

@@ -184,6 +184,7 @@ Docking+Viewports Branch:
 - Docking, Style: Docked windows honor display their border properly. (#2522)
 - Docking: Fixed incorrectly rounded tab bars for dock node that are not at the top of their dock tree.
 - Docking: Fixed single-frame node pos/size inconsistencies when window stop or start being submitted.
+- Docking: Prevent docking any window created above a popup/modal. (#4317)
 - Viewports: Made it possible to explicitly assign ImGuiWindowClass::ParentViewportId to 0 in order
   to ensure a window is not parented. Previously this would use the global default (which might be 0,
   but not always as it would depend on io.ConfigViewportsNoDefaultParent). (#3152, #2871)

imgui.cpp

@@ -14897,12 +14897,23 @@ static bool DockNodeIsDropAllowedOne(ImGuiWindow* payload, ImGuiWindow* host_win
         return false;
     }
 
+    // Prevent docking any window created above a popup
+    // Technically we should support it (e.g. in the case of a long-lived modal window that had fancy docking features),
+    // by e.g. adding a 'if (!ImGui::IsWindowWithinBeginStackOf(host_window, popup_window))' test.
+    // But it would requires more work on our end because the dock host windows is technically created in NewFrame()
+    // and our ->ParentXXX and ->RootXXX pointers inside windows are currently mislading or lacking.
+    ImGuiContext& g = *GImGui;
+    for (int i = g.OpenPopupStack.Size - 1; i >= 0; i--)
+        if (ImGuiWindow* popup_window = g.OpenPopupStack[i].Window)
+            if (ImGui::IsWindowWithinBeginStackOf(payload, popup_window))   // Payload is created from within a popup begin stack.
+                return false;
+
     return true;
 }
 
 static bool ImGui::DockNodeIsDropAllowed(ImGuiWindow* host_window, ImGuiWindow* root_payload)
 {
-    if (root_payload->DockNodeAsHost && root_payload->DockNodeAsHost->IsSplitNode())
+    if (root_payload->DockNodeAsHost && root_payload->DockNodeAsHost->IsSplitNode()) // FIXME-DOCK: Missing filtering
         return true;
 
     const int payload_count = root_payload->DockNodeAsHost ? root_payload->DockNodeAsHost->Windows.Size : 1;