How do I disable whatever X-window-system-wide setting is intercepting Ctrl-Period for some kind of special input popup?
More detail: I'm trying to bind an emacs command to Ctrl-Period (C-.), which worked fine on a previous system. On this new computer there seems to be something going on at the X level (xinput? xkb?) that's intercepting the keystroke so that emacs (or most other applications) simply aren't seeing it.
Instead, when I press Period while holding Ctrl, a small underlined "e" appears, either in-line when I'm in a graphical window, or as a popup on a terminal or emacs window (see images below). If I type further characters, they show up after the underlined "e", also underlined. The later characters can be backspaced, but the "e" cannot, and remains until I press either Enter, Esc, or a second Ctrl-Period. In the latter two cases (a second C-. or Esc) all the underlined characters disappear with no other effect, while in the former case (Enter) they disappear but immediately send the following keystrokes.
This is clearly happening at the window-system level because if I press C-. in one application and then move my cursor to another to type, the popup jumps between windows to whichever has focus (with sometimes odd side effects).
Finally, some applications do see the keystroke: if I add an key event listener to a blank webpage in Chrome, as long as I don't have an input element focused, the key event is emitted exactly as I'd expect. But if I have an input element focused, then weird "unidentified" keys are emitted while the underlined "e" is showing.
Result of ctrl-period (followed by "123") in browser location bar:
Result of ctrl-period (followed by "456") in terminal, emacs is similar: