Clicking the close button on the top-right of the drive-app does not close the app in mustash.

The window captions for the drive-app are drawn by the app itself. I suspect it tells about that, and sends a 'draggable region' to the window server. Window server/manager uses that information to drag the window, but never dispatches the events to the clients. As a result, you can press on the close button and drag to move the window, but you can't click on it to close the window.