Improve native browser search support in full windowed mode (hidden: until-found)

[krassowski]

Problem

When searching notebooks in full windowing mode by using the native browser search function (rather than JupyterLab search box) not all results are shown because the cells which are out of view are not rendered (these are either detached or hidden using display: none.

Proposed Solution

The HTML spec now includes a hidden: until-found attribute which corresponds to content-visibility: hidden but is included in search results and revealed when user iterates through the browser search results. For an example usage see MDN.

For now this is only implemented in Chrome/Edge

• Firefox https://bugzilla.mozilla.org/show_bug.cgi?id=1761043
• Safari https://bugs.webkit.org/show_bug.cgi?id=238266

Additional context

• the use of content-visibility for windowed notebooks is discussed in Use content-visibility with contain-intrinsic-size for size caching in windowed notebooks #16327; this issue imposes a constraint that we should use content-visibility: hidden rather than content-visibility: auto if we want to later swap it for hidden: until-found as it is equivalent to the former
• independently, the JupyterLab search can be improved by making it use the new highlights API once widely available; this will make the UX of search more closely follow the native browser search experience, at least on the presentation layer Migrate to CSS Custom Highlight API for search results #15333