Drag and Drop fails in Standalone and Packaged builds when WidgetComponent’s Widget is set at runtime

When a UWidgetComponent has its widget assigned dynamically at runtime using CreateWidget and SetWidget, drag & drop operations fail in Standalone Game and packaged/game builds, while working correctly in PIE.

If the WidgetClass is assigned directly on the WidgetComponent in the actor placed in the level (so the widget is created during component initialization rather than via SetWidget), drag & drop works correctly in all modes.

The issue is caused by the runtime behavior of UWidgetComponent::RegisterWindow(). The SVirtualWindow created for the component is not properly parented to the game viewport when the widget is swapped dynamically.

The problem was reproduced in 5.6, 5.7 and Main CL 48883915.

A workaround was suggested to the licensee in the UDN case (adjusting the early-return logic in RegisterWindow() so that game worlds in editor builds do not skip the parent assignment).

A repro project (built in 5.7) will be attached.

1. Open the repro project.
2. Load the map: L_Test.
3. Press Play in PIE and attempt to drag the widget rendered by the WidgetComponent. Drag and drop works as expected in PIE.
4. Now launch the same level in Standalone Game from the editor (Play → Standalone Game). Attempt to drag the widget again.

Result: Drag and drop does not work in Standalone. Issue persists in a source build from Main CL 48883915.