Investigate `headed` differences in the UI tests for Firefox

[jtpio]

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In https://github.com/jupyter/notebook/pull/6872 we noticed some differences for the rendering of the "Click to add a cell" button in Firefox when running the Playwright tests.

default (on CI)

The "Click to add a cell" button is displayed even when not actively hovering on it.

--headed

The "Click to add a cell" button is not displayed, which is the expected behavior.

We should consider investigating this more or report an issue on https://github.com/microsoft/playwright. Or check again later if the workaround can be removed in a future Playwright version:

https://github.com/jupyter/notebook/blob/c77ebdcfbf338a457b6119ac0be0cf344ab734bf/ui-tests/test/utils.ts#L38-L48