[Linux/X] CSS editor: text selection is not setting the primary selection

[dsalt]

Selecting text normally sets the primary selection. This is not happening within the CSS editor.

Pasting the primary selection content via middle-click, however, is working properly. The failure to set the primary selection means that I'm about to paste something other than intended.

(Copying and pasting via the clipboard works normally, but that's not what I usually use.)

[tophf]

Pasting and selecting is handled by CodeMirror and we use the old version 5. I don't know if it may be augmented/fixed, someone should investigate it. Eventually, we should switch to another engine, maybe CodeMirror v6, but that's a huge task as there's a ton of things to rewrite.