Select-to-copy on Linux does not work

User in 3344129-zen reports that they used to be able to select text to copy to clipboard, now it does not work.

### Expected
Selecting text copies it.

### Observed
> Just selecting text with the mouse does not allow me to paste said text into other applications with the middle mouse button, like it used to do, and like all other applications do…

We discussed with the user and clarified that they are able to successfully copy using any way *other* than selection (Edit->Copy in the menu and Ctrl+C both work), and that they are able to successfully paste using the middle mouse click. The only thing not working is that you can't select to copy.

No file, test, or entry point is named. First reproduce the selection-to-middle-click workflow on Linux and trace the app’s text-selection and clipboard handling; done means selecting text makes it available to other applications via middle-click while Edit→Copy, Ctrl+C, and middle-click paste remain working.

Written by the indexing model from the issue text.