Flaky UI tests

[jtpio]

It looks like we still have some flakiness with the notebook toolbar sometimes missing the overflow button menu:

cc @brichet who looked into the notebook toolbar recently (as part of #7311) and may know if that might also be happening in JupyterLab.

Originally posted by @jtpio in #7361 (comment)

[brichet]

Thanks @jtpio for the ping.

Wondering if this is only a matter of time for the button to be visible, and if we want to fix it (if it's inconvenient to use) or only fix the test (waiting for the button to be visible).
Did you notice the missing button outside the tests?

[jtpio]

Did you notice the missing button outside the tests?

Usually not, so it might indeed just be an issue with the tests.

There is some logic to wait for the kernel status animations to be gone before taking the screenshot, which may not be enough:

notebook/ui-tests/test/mobile.spec.ts

Lines 58 to 59 in 08fe5c5

	// wait for the kernel status animations to be finished
	await waitForKernelReady(notebook);

But maybe there could indeed also be a check to wait for that button to be visible, just to be sure.

[brichet]

I can take a look at the test and we'll see if it's enough.

[brichet]

Probably related to #6731, see #7371 for details.

[jtpio]

Re-opening to keep track of updating to a new JupyterLab version, and check if it fixes the flakiness.

[jtpio]

From #7378 (which should include jupyterlab/jupyterlab#16335), it looks like this time it is the button to toggle the virtual scrollbar that is sometimes showing up:

Actual	Expected