Keyboard repeat lag on GNOME

[kkga]

Godot version:
3.1.1 stable, also happens on 3.2 dev

OS/device including version:
OS: Linux, Fedora 30, GNOME 3.32.2
GPU: Intel UHD 620

Issue description:
When using the script editor on GNOME desktop, the response to keyboard repeat is “lagging”.
It’s difficult to illustrate with a GIF, but simply put: if you hold a key (e.g. arrow down to scroll) and then release it, the release triggers not immediately but after a certain lag.
Response to initial keystroke is immediate, so the lag only happens on keyboard repeat, i.e. when holding a key.

This happens only on GNOME, both when running on Wayland and Xorg.
I’ve tested on i3 and Sway: the problem disappears.

Adjusting keyboard repeat rate in GNOME also affects this issue but doesn’t solve it: higher repeat rate makes the lag in Godot longer. So if I have a very high repeat rate, Godot might keep “firing” the repeated keystroke for over ~2sec after I release the key.

To exclude the possibility that it's specific to my machine and OS setup, I’ve also tested and confirmed the issue on a spare laptop with a clean install of Fedora 29.

[kkga]

After some more experimenting found that the lag is directly tied to my screen resolution, as I'm using a 4k display: if I switch the resolution to 1440p or 1080p -- the lag disappears.

UPD: another observation: resizing the window to something smaller than fullscreen also fixes the issue, even on 4k resolution.

[Calinou]

@kkga What framerate do you get in the editor? You can start the Godot binary with --print-fps command-line argument to print the FPS continuously to standard output. Note that this setting isn't passed to the editor when starting the project manager, so you will have to edit a project directly by specifying a path to a project.godot file:

/path/to/godot.binary ~/Documents/Godot/some_project/project.godot --print-fps

If the framerate is too low, not much can be done about it, except disabling V-Sync in the Project Settings (which will also impact the editor after restarting it). We could add support for adaptive V-Sync in the long term, so that low FPS won't increase input lag and stuttering as much.

[kkga]

The framerate is really bad :/

This is with V-sync turned off.

Here's what it's like when navigating the script editor in fullscreen:

The only way to get a usable framerate is to resize the window to something like this:

---

As I mentioned before, I'm only experiencing such performance drop on GNOME.

[Calinou]

As I mentioned before, I'm only experiencing such performance drop on GNOME.

GNOME's compositing window manager likely doesn't help with rendering performance… This is why you get better performance in a window manager with compositing disabled 😉

Still, it will likely improve when text batching is implemented (see #19917).

[KoBeWi]

Can anyone still reproduce this bug in Godot 3.2.3 or any later release?

[kkga]

I have a discrete GPU now, so I can't really test under the same conditions.