- while nothing is shown, the whole view is a button that opens the file
  dialog ("Open a CityGML file / or drop it here")
- files can be dropped on the view at any time; an overlay shows the drop
  target, a drop replaces the shown file like choosing one
- drops elsewhere on the page are ignored instead of making the browser
  leave the viewer

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>