RFR: JDK-8308659: Use CSS scroll-margin instead of flexbox layout in API documentation [v2]

[Hannes Wallnöfer]

> A few years ago we switched to [Flexbox Layout](https://developer.mozilla.org/en-US/docs/Glossary/Flexbox) to implement the sticky header in the API docs generated by `javadoc`. This solved the problem of anchor link targets [not being positioned correctly](https://bugs.openjdk.org/browse/JDK-8223378), but it also introduced a few new ones:
> 
>  - It required a workaround to get browser history to work (JDK-8249133, JDK-8250779, 8286832)
>  - It changed certain aspects of scrolling behavior in the browser (JDK-8301080)
>  - It changed the way some CSS properties are interpreted (JDK-8315800)
> 
> The reason for most of these problems is that the layout paradigm used by Flexbox is very different from traditional layout of HTML pages. The `scroll-margin-*` CSS properties introduced by the [CSS Scroll Snap Module](https://www.w3.org/TR/css-scroll-snap-1/) provide a simpler and less intrusive way to implement link target offsets in combination with sticky elements implemented using [`position: sticky`](https://developer.mozilla.org/en-US/docs/Web/CSS/position). However, although it is implemented [in all supported browsers](https://caniuse.com/?search=scroll-margin), it comes with its own challanges and quirks, which are explained below.
> 
> The first problem to overcome was that `position: sticky` is more fragile on mobile browsers than Flexbox. If some part of the page content overflows the allotted horizontal space, the whole page can be zoomed out to view the whole content. This causes the header to become unsticky and the link target offsets to become erroneous. It was therefore necessary to make sure page content never overflows its allotted horizontal space, either by adding line break opportunities, or by making all or part of the page horizontally scrollable when its content overflows. Line break opportunities are difficult to add especially in  preformatted code, so I opted for making the parts of the page most likely containing long lines of code scrollable. 
> 
> The next problem was that enabling horizontal scrolling on an element or one of its containing elements breaks the `scroll-margin-top` property in Chrome due to a browser quirk (both desktop and mobile versions). This means that the scrolling must occur in a child element rather than the sections or other elements serving as link targets. 
> 
> When enabling horizontal scrolling on the contents of sections containing user-provided content, another problem is that it disables [Margin Collapse](https://www.joshwc...

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  A few more tweaks
  
   - Move scroll-margin update from search.js to script.js
   - Use DOMContentLoaded event to update scroll-margin
   - Avoid use of :has() CSS pseudo-class which is unsupported on Firefox

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15969/files
  - new: https://git.openjdk.org/jdk/pull/15969/files/fcfcc5fe..aad89904

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15969&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15969&range=00-01

  Stats: 12 lines in 3 files changed: 7 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15969.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15969/head:pull/15969

PR: https://git.openjdk.org/jdk/pull/15969