Block inserter lacking a clear close button

[getdave]

As reported in #46939 (comment) the block inserter is lacking a clear close button. This makes it very difficult for users of assistive technology to exist the popover.

A clear button should be provided which closes the inserter and returns focus to the last focused element.

[draganescu]

This seems to be a feature request for the quick inserter.

[alexstine]

Yes but a must none the less. Not sure how I missed this elsewhere. Maybe I just never used it anywhere else before now. Escape is not a given sadly so having a close button makes it clear.

[carolinan]

I think this needs some design guidance before it can be implemented.

[jasmussen]

Related to a conversation here, I'd like to hear thoughts on having it be an on-focus close button that appears, for example, at the end of the list of tabbables.

[ciampo]

As @jasmussen suggests, could we add a visually hidden button that can be focused via keyboard navigation? The button could become visually visible when focused, similarly to the "skip to content" links that are usually added at the top of a web page.

[getdave]

I implemented a proof of concept in #47327