[v4] Setting selection while opening context menu causes deletion

[njx]

1. In demo/theme.html from v4, add this code to the end of the <script> at the bottom:

  editor.getWrapperElement().addEventListener("contextmenu", function () {
      editor.setSelection({line: 0, ch: 0}, {line: 1, ch: 0});
  });

1. Open the file in the browser and right-click in the editor

Result: The first line is deleted. This doesn't happen if you handle the context menu from CodeMirror's own contextmenu event.

[marijnh]

Attached patch helps for me. Please verify.

[njx]

Verified, thanks.