Originally Posted by o_parallos
How do I ctrl-C in mrxvt, or, for that matter, any control sequence?
On-screen keyboards don't have the "modifier" keys available on hardware keyboards. On a real keyboard holding the shift key and pressing a returns <shift>a, which X11 interprets (by default) as "A". On an on-screen keyboard, there is no way to hold one key while tapping another and instead tapping <shift> brings up essentially a different keyboard and then tapping A returns "A" directly (not <shift>a).
The same thing is the case for other modifiers, including <ctrl>, but the iLiad keyboard does not have <ctrl>.
The only alternative is remapping individual keys. This could be done (for all apps) at the X11 level, but mrxvt has its own shortcuts/macros capability which allows keys to be remapped just for mrxvt. There are several keys (EURO, <shift>EURO, <shift>2, and many <aeu> and <aeu><shift> keys) that have no effect in mrxvt. However, I have not been able to work out how to identify these for mrxvt macros. The only other "unused" keys are SELECT, DELETE, HOME, END (by default: SELECT is <ctrl>A, and END is <end> but works as ESCAPE). Version 4 of mrxvt (in post #37) remaps these four keys to <ctrl>C, <ctrl>Z, QUIT, and <escape>. Edit the file mrxvtrc to change these mappings. I am not sure that ^C and ^Z are working, and QUIT is perhaps too close to the relatively widely used END for comfort, so let me know if you have a better layout.
If the hardware keys were associated with mrxvt, then all their long keypresses would be available as mrxvt macros and the up key could be mapped to QUIT as usual.