Investigate `headed` differences in the UI tests for Firefox

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

Start with ui-tests/test/utils.ts lines 38-48 and reproduce the Firefox Playwright tests in default and --headed modes. Compare the rendering of the “Click to add a cell” button, then determine whether the workaround can be removed, should remain, or warrants a Playwright issue.

Written by the indexing model from the issue text.