Mozilla (only) — Input cells do not accept X selection pasting

If I select some text, say in a XTerm or Emacs, dragging with mouse button 1, the selected text (X selection) can be pasted inside a well behaved X client using a mouse button 2 click:

- this procedure works OK if I open a notebook using Chrome, I can paste the X selection using a button 2 click in any input cell,
- this procedure works OK in Mozilla if I paste in a generic text box,
- this procedure DOES NOT work in Mozilla when I try to paste into an input cell of a notebook.

This is annoying because certain applications (e.g., XTerm...) do not provide an immediate method to copy the X selection to the system clipboard.

Forgot to mention, I see this behaviour on Linux running XOrg X server.

Reproduce the issue on Linux with XOrg using Mozilla and a Jupyter notebook, comparing a generic text box with an input cell. Start by tracing how input-cell paste and mouse-button-2 events are handled in the notebook frontend. Done means an X selection can be pasted into an input cell in Mozilla without regressing the working Chrome behavior.

Written by the indexing model from the issue text.