Middle-button-click doesn't paste in Linux

[lukehutch]

Describe the bug In Linux, pasting the most recently-selected text, by clicking the middle button, doesn't actually paste anything.

To Reproduce Select text in one window; middle-click in gdbgui window. Nothing happens.

Expected behavior Middle button should paste from secondary clipboard.

Please complete the following information:

• OS: Fedora 33
• gdbgui version (gdbgui -v): 0.14.0.2
• gdb version (gdb -v): N/A
• browser [e.g. chrome, safari]: Chrome
• python packages (pip freeze): N/A