"JS Error in CoverflowAltTab: TypeError: this._subSwitchers is null on Alt+Tab"

[anasgets111]

Describe the bug
When using the CoverflowAltTab extension to switch windows (e.g., using Alt+Tab), I intermittently encounter a JavaScript error in the GNOME Shell logs. The error message reads:

JS ERROR: TypeError: this._subSwitchers is null
    _activateSelected@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:943:34
    _keyReleaseEvent@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:878:22
    @resource:///org/gnome/shell/ui/init.js:21:20

This error appears repeatedly during window switching and may cause the window switcher interface to freeze or behave unexpectedly.

To Reproduce

1. Ensure the CoverflowAltTab extension is enabled.
2. Use the window switcher (e.g., press Alt+Tab) to cycle through open applications.
3. Observe the GNOME Shell logs (using journalctl or similar) to see the error messages.

Expected behavior
The CoverflowAltTab extension should display open windows in a cover-flow style without generating any JavaScript errors. Window switching should be smooth and error-free.

Logs
I captured logs using the following command:

journalctl -f -o cat /usr/bin/gnome-shell /usr/bin/gjs

A snippet of the output is shown below:

fadeInOut true scaleUpDown false
JS ERROR: TypeError: this._subSwitchers is null
_activateSelected@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:943:34
_keyReleaseEvent@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:878:22
@resource:///org/gnome/shell/ui/init.js:21:20

Your Environment

• Extension version:
  • Version: 76
  • Operating System:
  • Arch Linux
• GNOME Shell Version:
  • GNOME 47.3 (Wayland)
• Are you using other GNOME Shell extensions?
  • Enabled extensions:
    • user-theme
    • extension-list
    • middleclickclose
    • drive-menu
    • appindicatorsupport
    • arch-update
    • dash-to-panel
    • gsconnect
    • weatheroclock
  • I have tested the issue with only CoverflowAltTab enabled, and the error still occurs.

Additional context

• My system information is as follows:
  • CPU: AMD Ryzen 9 5900X
  • GPU: GeForce RTX 3080 (NVIDIA open-modules driver, version 565.77)
  • Kernel: Linux 6.12.10-arch1-1
  • Display: Dual monitors (Samsung 27" 2K at 180Hz and Dell 24" 1080p at 60Hz)
  • Shell: fish 3.7.1
• Any help to resolve this issue is appreciated.

[dsheeler]

Thank you for the report! I didn't know this was happening, and I don't see it yet on my machine, so far. I think I made some code changes that might have solved this, but I'm not sure. Can you get the latest version v77 from github and try it?

[dsheeler]

If you want to wait, it's making it's way through the review process and when that's done, it will be installable through the web interface.

[anasgets111]

OK got the update,it's still happening, let me know what info you need me to provide ?
this is a quick video

Screencast.From.2025-02-08.12-26-43.webm

[dsheeler]

Thanks for the video. Are those messages logged after an event? Also, the switcher basically isn't working at all for you, right? I've never seen this situation!

[anasgets111]

I confirmed that this null exception happens only when I alt+tab fast such that the animation doesn't have time to complete; the switcher sometimes fails to display properly.
However, when I don't switch as rapidly and the animation completes, no exception and no stuttering.

Below is a sample of the log output when the error occurs:

fadeInOut true scaleUpDown false
meta_window_set_stack_position_no_sync: assertion 'window->stack_position >= 0' failed
surface_state_changed: assertion 'wl_window->has_last_sent_configuration' failed
fadeInOut true scaleUpDown false
fadeInOut true scaleUpDown false
fadeInOut true scaleUpDown false
JS ERROR: TypeError: this._subSwitchers is null
_activateSelected@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:943:34
_keyReleaseEvent@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:878:22
@resource:///org/gnome/shell/ui/init.js:21:20
fadeInOut true scaleUpDown false
JS ERROR: TypeError: this._subSwitchers is null
_activateSelected@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:943:34
_keyReleaseEvent@file:///home/anas/.local/share/gnome-shell/extensions/[email protected]/switcher.js:878:22
@resource:///org/gnome/shell/ui/init.js:21:20

Hope this helps in narrowing down the problem!

Screencast.From.2025-02-08.19-46-04.webm