Intent to Ship: The new white-space normalizer of HTMLEditor

Summary: HTMLEditor normalizes collapsible white-spaces which user typed to make all of them visible, i.e., as not collapsed.

Currently, our builtin editor keeps traditional behavior since it's initially implemented. That is, allow to puts an ASCII white-space at last of the white-space sequence and convert its previous one as an NBSP. If it's the last white-space immediately before a block boundary, HTMLEditor puts a <br> element to make it visible (this rendering result is also incompatible with the other browsers).

On the other hand, the other browsers puts an NBSP to the last white-space of a white-space sequence and always normalizes the white-space sequence with pairs of an NBSP and an ASCII white-space (the order is opposite between Chrome and Safari, though). This behavior is more stable and simpler than ours because web apps may delete following things of typed white-space without maintaining that it stays visible (This actually happens in Gmail).

Therefore, we should follow their behavior, and I think it's reasonable to make us behave as Chrome and actually I implemented the new normalizer which is almost compatible with Chrome. (Unfortunately, in some edge cases, we get different result whether normalize or does not do that for surrounding white-spaces at editing range.)

In the following bug, I'll enable the new behavior for the nightly builds and early beta builds. Then, I'll enable them in all builds after watching regressions reports in a couple of releases.

(The legacy paths will be deleted once it's shipped in all channels soon.)

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1951832

Specification: N/A

Platform coverage: All

Preference: editor.white_space_normalization.blink_compatible

web-platform-tests: https://wpt.fyi/results/editing/whitespaces/chrome-compat?label=master&label=experimental&aligned

NOTE: For web developers, I strongly recommend you to use `white-space: pre-wrap` or something to prevent the white-space normalization since users cannot get same result as they typed if they typed NBSPs because NBSPs are treated as ASCII white-spaces if white-spaces are collapsible (e.g., when `white-space: normal`, `white-space: pre-line` and `white-space: nowrap`).

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Masayuki Nakano <masa...@d-toybox.com>
Working on DOM, Events, editor and IME handling for Gecko

We'll ship this new behavior in all channels starting with 139.

According to the reported bugs, this will fix a lot of web-compat issues on Office 365.

Note that once this is shipped in the release channel, I'll delete the legacy normalizer soon because it wastes the binary size and makes the code harder to read.

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