Solution:
* to have Ctrl+1 working, the x-input method has to be better than xim;
commonly one has to e.g. install ibus-gtk and ibus-gtk3, and remove .xinputrc
(if present, since it may contain old "run_im" setting). if it would not still
work (due to some old defaults settings), im-config may b
It is the same way for "Ctrl+1" key input (meaning "insert text cell");
and may be for some other combinations too. Ctrl+U seems to not work at
all.
Alt-return (for full-screen mode) seems to start to work after once
going to full screen. Notice that in full-screen mode, the "Ctrl+1" for
text cell
