Toolbar is often empty when opening a Notebook

[jtpio]

Sometimes the notebook toolbar moves most of the items in the toolbar popup, leaving only the popup opener visible.

This is one of the causes for the flaky UI tests (example run):

condensed-toolbar.mp4

But this sometimes also happens when opening a notebook or refreshing the page:

There seems to be a race condition somewhere. Or the toolbar resize logic is being triggered too early in the process of loading the notebook widget and adding it to the shell.

[jtpio]

This looks like a real issue / race condition.

Part of the fix should include the removal of this workaround in the UI tests added in #6487:

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

Lines 21 to 23 in 62c0fa8

	// temporary workaround to trigger a toolbar resize
	// TODO: investigate in https://github.com/jupyter/notebook/issues/6553
	await page.setViewportSize({ width: 524, height: 800 });

and:

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

Lines 44 to 46 in 62c0fa8

	// temporary workaround to trigger a toolbar resize
	// TODO: investigate in https://github.com/jupyter/notebook/issues/6553
	await page.setViewportSize({ width: 524, height: 800 });

[andrii-i]

I'm having a hard time reproducing this locally. Does this happens locally (often) or this is more of a testing problem? If it's the latter, should this issue be moved to 7.x and not be a blocker?

[jtpio]

Right, as noticed in #6887 (comment), the issue now seems to be more about some items being cropped rather than items missing from the toolbar:

However it seems to be reproducible when testing with a mobile layout via the dev tools and refreshing the page several times:

items are "cropped"

expected behavior

[jtpio]

Given that this is not very easily reproducible (although the UI tests seem to be catching it consistently in #6887), we can bump that one to 7.0.x for now.

But if someone would like to work on it in the meantime please feel free.