Stop cleanUpNbsps orphaning CKEditor's zero-width filling char in titles (BL-16808)

[andrew-polk]

Problem. Follow-up to #8305. A tester found that on 6.4 a title typed as "One Two" with Enter (or Shift+Enter) pressed at the start of "Two" still reaches the Publish tab and bloomlibrary.org as "OneTwo", even though #8305 now puts a line break between the two lines. Show Hidden Characters reveals a zero-width space at the start of the second line. 6.5 does not have the problem.

Cause. CKEditor plants a zero-width "filling char" (U+200B) at the caret whenever it re-selects a caret that sits next to an inline element, such as the line-break span Shift+Enter inserts, and removes it later by holding a reference to that text node. Bloom's half-second tidy-up after every keystroke took a CKEditor bookmark and restored it, which is such a re-select, and it also assigned the box's innerHTML whether or not anything had changed. Each rewrite detached the node CKEditor was tracking, so the character stayed as ordinary text, a fresh one was planted on the next pause, and the result was saved. Chromium renders a newline adjacent to U+200B as nothing at all, so the line break became invisible. Reproduced in a running 6.4 build over CDP, and master fixed both halves in BL-16558 and BL-16717, which is why 6.5 is clean.

What the PR does. Ports the two master changes: cleanUpNbsps assigns innerHTML only when it actually converted a non-breaking space, and the tidy-up takes a bookmark only when a tool is active or the box holds a comment or an nbsp, the only cases in which anything rewrites it. With both, the tester's Enter and Shift+Enter sequences are clean at every step in a running Bloom, Show Hidden Characters included, and the saved title is "One \nTwo". Five new tests check node identity and the gate. Not done: cleaning books that already carry a stray zero-width space, and stripping one that CKEditor itself plants after, say, a Backspace next to a line break and that Show Hidden Characters then orphans; master strips those at save (BL-16490), which was judged too risky for 6.4 because Khmer, Thai and Myanmar text uses U+200B legitimately.

Ref: https://issues.bloomlibrary.org/youtrack/issue/BL-16808

Devin review

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This change is 

[andrew-polk]

[Claude Fable 5.1] Consulted Devin on 2026-09-08 20:27 UTC up to commit 3037a63ac8cbd7f7cfb4710a69395e110c53c22a. It raised one non-severe bug and one Investigate flag, both about the same accepted limit (the box is still rebuilt on the rare pause where a non-breaking space is converted); each has its own resolved thread above with the reasoning. No informational items. CI passed.

[andrew-polk]

[Claude Fable 5.1] Consulted Devin on 2026-09-08 21:11 UTC up to commit 32b5e5e168d186fff43dc0da72277472ee520a6c (and on the two commits before it this run). Each re-review repeated the same non-severe bug and Investigate flag about the accepted limit already answered in the resolved threads above (the box is still rebuilt on the rare pause where a non-breaking space is converted); nothing new. No informational items. CI passed.

[andrew-polk]

[Claude Fable 5.1] Consulted Devin on 2026-09-08 21:48 UTC up to commit cd867a2bfd3edd62bf5a5a3f9f07748bf91b624c (comment-wording change). Same two repeated findings as before, already answered in the resolved threads above; nothing new.

[StephenMcConnel]

@StephenMcConnel reviewed 2 files and all commit messages.
Reviewable status: all files reviewed, 2 unresolved discussions.

[StephenMcConnel]

@StephenMcConnel resolved 2 discussions.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on andrew-polk).