(Ctrl+)Shift+Ins shortcuts are not strict
Ctrl+Shift+Ins is recognized by vte and used for pasting the clipboard contents; Shift+Ins pastes the selection.
However, these shortcuts are not "strict" - meaning that they allow other modifiers, such as Alt. Alt+Ctrl+Shift+Ins will still paste the clipboard, and Alt+Shift+Ins will paste the selection.
I don't think this implementation is correct. I would prefer that adding other modifiers made the shortcuts invalid. The relevant code is here: https://gitlab.gnome.org/GNOME/vte/blob/master/src/vte.cc#L4764
Edited by Igor