Ghost placeholder + sidebar descriptor for moved blocks

[adamsilverstein]

Closes #78409. Stacked on #77979 (base branch suggest-mode-6c-move-ui).

What

Follow-up to the structural-suggestion stack, addressing review feedback from @saroshaga on #77979: a block-move suggestion previously only decorated the block at its new position, and the sidebar text was vague ("Moved <block> from a previous block to a different block").

This PR adds:

1. Ghost placeholder at the original position — a non-interactive, struck-through "ghost" of the moved block (block icon + title + one-line excerpt) rendered where the block came from, tinted with the suggester's avatar color via the existing --suggestion-author-color variable so it visually pairs with the decorated block at the destination.
2. Meaningful sidebar descriptor — BlockMoveDiff now computes direction/distance from the captured fromIndex vs. the live index: "Moved up 3 blocks" / "Moved down 1 block" for same-parent moves, "Moved into Group" / "Moved to top level" for cross-parent moves.

How

Presentational only — no block-editor store mutation, nothing syncs via CRDT. The block continues to exist exactly once in the tree.

• move-ghost-index.js — pure buildMoveGhostIndex turning pending-move markers into an { after, before } anchor index (null-anchor / first-child and missing-anchor fall back to the old parent's first sibling; empty old parent → no ghost).
• overlay-context.js — a single O(n) scan in SuggestionOverlayProvider exposes the index on context (a stable signature string keeps useSelect referentially stable; the index rebuilds via useMemo only on placement-affecting changes), avoiding an O(n²) per-block scan.
• with-suggestion-overlay.js — the existing withSuggestionBlockClassName HOC renders <SuggestionMoveGhost> siblings around the anchor block.
• suggestion-move-ghost.js — the presentational ghost (aria-hidden, pointer-events: none, never in the block list).
• content-suggestion.scss — canvas-loaded ghost styling reusing the stack's suggestion-color tokens.
• suggestion-diff.js — exported pure describeMove + rewritten BlockMoveDiff.

Out of scope (deferred)

A drawn connector/arrow between the ghost and the destination is intentionally not included — it needs an SVG overlay spanning two DOM positions inside the scrolling iframe canvas (reposition on scroll/resize), a separate risk profile. Tracked for later evaluation in #78409.

Testing

• 170 suggestion-mode unit tests pass (15 new: pure index builder, ghost component, descriptor function, HOC ghost rendering). Lint + stylelint clean. Production build clean.
• Manual matrix to verify (Suggest mode, login admin/password): move a paragraph up/down within a parent; move the first child out of a Group; move a top-level block into a Group; two moves in one doc; confirm a second peer sees no extra block and serialized content is unchanged.

Refs #77434, #77979.

[github-actions]

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[github-actions]

Size Change: +1.46 kB (+0.02%)

Total Size: 7.52 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	487 kB	+1.09 kB (+0.23%)
build/styles/block-editor/content-rtl.css	5.92 kB	+92 B (+1.58%)
build/styles/block-editor/content-rtl.min.css	4.47 kB	+89 B (+2.03%)
build/styles/block-editor/content.css	5.92 kB	+94 B (+1.61%)
build/styles/block-editor/content.min.css	4.47 kB	+87 B (+1.99%)

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[saroshaga]

The move ghost placeholder worked for me in Playground: I could see the “Moved from here” placeholder at the original position and the dotted “Suggested move” treatment at the destination.

A few minor follow-up questions:

1. For ghost placement, could we confirm what happens when the block that used to be before the moved block has also moved? For example, if C is moved and its original previous sibling was B, but B is also moved elsewhere, would the ghost for C follow B to its new location? Should the ghost only use that previous sibling as an anchor if it is still in the original parent and is not itself pending-moved?

2. What should happen when the moved block was the only child in its old parent, for example moving the only paragraph out of a Group? Since there is no remaining sibling anchor, it looks like no ghost is rendered. That may leave reviewers without an original-position cue for that case.

3. For accessibility, the ghost is aria-hidden, and the destination “Suggested move” label is CSS-generated. Should there be a non-focusable accessible cue, such as a role="note" / visually-hidden label, so screen-reader users can also tell where the move came from?

[adamsilverstein]

Thanks for the testing and the sharp follow-ups - all three are addressed:

1. Ghost following a moved anchor - good catch, this was a real bug. buildMoveGhostIndex only checked that the anchor still existed, but a pending-moved previous sibling still exists, so the ghost trailed it to its destination. Anchor selection now excludes any pending-moved block (both the recorded previous-sibling and the first-child fallback), with the same exclusion mirrored in the overlay move-signature so the memo stays in sync. (44a4ef2d85e)

2. Only child / no surviving sibling - now handled. Added an insideParent fallback so the ghost renders just below the emptied old parent. It's skipped at the root and when the old parent is itself gone or pending-moved (anchoring there would drag the ghost along). One caveat: it renders adjacent to the container rather than literally inside it - the editor.BlockListBlock filter can't inject into an empty parent's inner-block region - but the empty container plus the "Moved from here" label keep the origin clear. (1f8deed3e62)

3. Accessibility - the ghost is now a role="note" with a visually-hidden "Original position of moved " sentence (the decorative label/excerpt stay aria-hidden to avoid a double read). The destination tab text also moved out of CSS content into a data-suggestion-move-label attribute (so it's translatable, not hardcoded English) and gained a visually-hidden destination cue. (3a489bd330a)

Each change has unit/HOC coverage. Thanks again!

[adamsilverstein]

Superseded by the fresh-stack restructure of Suggest mode (#73411).

The feature has been re-sliced from this 16-deep stack onto current trunk as a clean 2-PR stack:

• Suggest mode: core subsystem (intent modes, overlay capture, structural + attribute suggestions) #79713 — Suggest mode: core (intent modes, overlay capture, structural + attribute suggestions, REST/PHP, auto-save, sidebar, summary)
• Suggest mode: inline text and formatting suggestions via markers #79714 — Suggest mode: inline text suggestions (the <mark> marker layer)

The whole feature now sits behind a Suggestion Mode experiment (Settings → Experiments → Collaboration). Closing in favour of the new stack — details: #73411 (comment)