Stop gobbling escape key

[gabalafou]

Fixes #12811.

Subject: restore escape key functionality to HTML modal dialogs in Sphinx

Feature or Bugfix

• Bugfix

Purpose

• Restore default escape key functionality to modal dialogs, previously prevented by sphinx_highlight.js.

Detail

• When anything other than a link or the body is focussed, Sphinx should not handle the escape key.
• For rationale, see discussion in Cannot press escape key to close modal dialog #12811.

Relates

• Make overlay sidebars behave like modals pydata/pydata-sphinx-theme#1942

[jayaddison]

I'm not convinced by this - sometimes the document.activeElement (focus) may be set to other document-level elements (body, for example) and it seems valid to clear the highlighting in those cases.

This is useful, for example, when searching for a term in some Sphinx-generated HTML documentation, clicking on a result, and then wanting to clear the highlighting without having taking any other action on the page.

[gabalafou]

@jayaddison, apologies, my wording isn't clear. When document.activeElement === document.body that essentially means that nothing on the page has focus. In other words, document.activeElement is never null, so if no other element is the active element, then the body is the active element. So, with the code changes introduced by this PR, if focus is on the body, the escape key should clear the search result highlights from the page.

I will update the PR description to match this more accurately.

That said, you raise a good question. In this PR, links (and document.body) are the only elements that while focused allow the escape key to clear the highlights. For any other focusable element, this PR assumes that it might have some default action for the escape key, so the escape key is ignored and the search highlights are not cleared. But might there be other exceptions? Are there other focusable elements that should allow the escape key to clear the highlights?