1) You copy some text by selecting it and with CTRL-C to copy it to the proper variable. (or in some other cases CLTR-INS)
2a) Paste that text somewhere else with CTRL-V. (or in some other cases SHIFT-INS)
2b) Select some text to be replaced by what you copied at step 1.
... Right!
Now Linux has this additional copy & past feature that allows direct pasting of the last selected text with the middle mouse button. (although annoyingly its implemented slightly different in different text/code editors. ... Usual scintilla.org/SciTE related editors.)
... Needled to say Linux uses two separate variables to keep those two features from messing each other up.
So how did game dev's implemented things.
1) Select some text ... and it will directly overwrite whatever is already in the CTRL-C variable.
Ergo: The previous step 2b will always fail because you just lost the text you intended to past.
Hope it was not intentional ... because if it was its even worse.
Only checked against/with key-names as it don't makes sense to me it would only be used there. Yea: Vanilla client on Linux.