Galata stuck when waiting for JupyterLab to be ready on Firefox

[jtpio]

Description

UI tests time out when using Galata with the Firefox browser. This happens when loading the default JupyterLab page, and might be related to the waitForApplication logic.

Reproduce

• Create a new extension from the extension template: https://github.com/jupyterlab/extension-template
• Follow the instructions to setup the UI tests and install playwright
• Run jlpm test --browser firefox
• The tests time out

chromium

galata-chromium.mp4

firefox

galata-firefox.mp4

Expected behavior

The tests should run fine like on Chromium.

Context

This was first noticed in jupyterlite/jupyterlite#1019 (which tests on both chromium and firefox), but it is easily reproducible with the extension template.

This is related to #15088, which can help catch such issues.

[jtpio]

There was the same issue a little while ago with chromium if not on the latest versions of the @jupyterlab and @lumino pacakges. This seems to have been fixed in #14945 for chromium at least, but not for firefox.

cc @fcollonval who might know about this and also as the author of #14945,

[jtpio]

Tagging as regression as this was not an issue with the 3.5.x packages.