[Development] applications dropping keyboard input

René J.V. Bertin rjvbertin at gmail.com
Sat Mar 18 10:27:54 CET 2017


I'm seeing an issue with certain Qt5 applications on Linux that is probably caused by something on such a low level that it seems more appropriate to ask on the development than on the "interest" ML. I'm not sure if it's a Qt or rather a KF5 issue but I'm beginning to think it must be the former.

The symptom is that periodically applications stop responding to the keyboard. This is almost always while I'm active with the keyboard, I cannot recall for instance having come back to the system and find the keyboard muted in my absence.
Until yesterday I had only seen it in the Qt5-based Konsole application, i.e. the longest-running application with which I interact almost exclusively via the keyboard. Sometimes the whole app would be affect (i.e. all windows and tabs), sometimes only a single tab. In all cases the situation sorts itself when I restart the app (or simply open a new tab).

I've seen it too on a much smaller number of occasions the KDE4 Konsole.

Yesterday's episode gave me 3 new observations:

- it started after I had opened a document in a running KDevelop5 session, from a Konsole5 application. The keyboard input stopped after a few keystrokes, in BOTH KDevelop and all windows of the parent Konsole process. Until now I had only seen this in Konsole itself so I assumed it was a Konsole issue. Clearly it's happening on a lower level.
- banging long enough on the keyboard an occasional keystroke would get through.
- changing keyboard layouts had no effect (session-wide or in Konsole), nor had restarting KWin (why would it), a suspend/wake cycle or cycling among virtual consoles. I did end up with a wonky keyboard layout though; I use the "English (Macintosh)" layout and lost the AltGr deadkey function in all Qt apps but not in GTK-based apps (i.e. AltGr+e,e would give me ´e instead of é). Usually I get out of that particular situation by toggling the layout to and from Fr but that didn't help here and the French layout was instable too. I had to restart the X server to sort that out.

I *think* I never noticed this on my previous notebook but only on the current model, an Intel N3150-based Clevo W5108 . That means I've seen it with Qt 5.5 and later, and kernel 4.5.x and later.

Does this ring any bells? Any suggestions what I could look into or try next time this happens?
It happens infrequently enough (fortunately) and too randomly to make it appealing to run with qDebug output enabled but if there's nothing else I could try to activate it for Konsole only for a while. (This cannot be done for already running applications, right?)



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