[QTI] Add an inline-only mode to TipTapEditor for gap match choices #6193

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Overview

Gap match choices (qti-gap-text) only allow inline content in QTI, so the rich text editor used for them should only offer inline formatting. TipTapEditor needs a way to be limited to inline-only features for a particular field. Any non-inline content that still reaches it should be converted to inline, whether it comes from stored data that was already broken or from the clipboard, so a gap match item can't be saved in a shape the schema rejects. This builds on #6151 and starts after it merges.

Complexity: Medium
Target branch: unstable

Context

• TipTapEditor currently has one configuration: the full set of block and inline features, for every field that uses it.
• After Bring back underline, strikethrough and align support, and remove RTE link support #6151, the editor saves underline and strikethrough as <span style="text-decoration"> and alignment as style="text-align", links are switched off, and pasted styles are filtered to the allowlist. Pasted blocks are kept.
• The editor's Image and Small nodes are block nodes. Math is inline.
• The QTI editor doesn't have a gap match interaction yet (today it has choice, text entry, ordering and associate). This task gives that interaction an editor it can use.
• qti-gap-text allows only text and inline elements (XSD GapTextDType). The passage around the gaps is ordinary block content.

The Change

• TipTapEditor should support an inline-only mode that a field can switch on. In this mode the content is a single inline run, with no paragraphs, headings, lists or other blocks.

• In inline-only mode the editor should offer only these features, all of which qti-gap-text allows:

  Offered	Not offered
  Bold, italic	Format dropdown (headings, small text)
  Underline, strikethrough (from Bring back underline, strikethrough and align support, and remove RTE link support #6151)	Bullet and numbered lists
  Subscript, superscript	Code block
  Math	Text alignment
  Inline code	Images
  Undo / redo	Line breaks

• Features that aren't offered should be unavailable everywhere: toolbar, mobile toolbar, keyboard shortcuts and markdown-style input rules. Hiding the buttons isn't enough.

• No line breaks. qti-gap-text allows <br>, but a gap answer is dropped mid-sentence into the passage and shown as a chip in the answer pool, so a line break would split the passage line and make the chip hard to read. Enter, Shift-Enter and Mod-Enter insert nothing.

• Pasted content should be reduced to inline: blocks become inline text, line breaks become spaces, and allowed inline formatting and math are kept. Anything not allowed, images included, is dropped.

• Stored content that contains blocks or line breaks should load as inline content without errors and save back as valid inline content.

• The existing full editor stays unchanged for every other field, including the gap match passage.

Out of Scope

• Building the gap match interaction editor itself (the passage, gap widgets and distractors list). This task only provides the inline-only editor mode it will use.
• Images in gap choices (qti-gap-img, or inline images in qti-gap-text). In the design, gap answers are typed as text straight into the passage's gap widgets, so image distractors wouldn't make sense either.

Acceptance Criteria

General

• TipTapEditor has an inline-only mode that a field can switch on; fields that don't switch it on work exactly as before
• In inline-only mode the toolbar and mobile toolbar offer only bold, italic, underline, strikethrough, subscript, superscript, math, inline code, and undo/redo
• In inline-only mode, keyboard shortcuts and markdown-style input rules can't create blocks (paragraphs, headings, lists, code blocks) or images
• In inline-only mode, Enter, Shift-Enter and Mod-Enter insert nothing, including on touch keyboards
• Pasting blocks keeps their text, allowed inline formatting and math as a single inline run, with each line break turned into a space; images and other disallowed content are dropped
• Stored content that contains blocks or <br> loads without errors and saves back as inline content with no <br>
• Content saved from inline-only mode validates as qti-gap-text content

Accessibility and i18n

• Toolbar options that aren't available are left out, not shown disabled, so screen readers don't announce controls that do nothing

Testing

• Tests cover the offered and removed toolbar options, Enter and Shift-Enter, pasting block content and line breaks, and loading stored block content in inline-only mode

References

• Bring back underline, strikethrough and align support, and remove RTE link support #6151, which brings back underline, strikethrough and align and removes links
• QTI 3 item schema, GapTextDType and GapMatchInteractionDType (contentcuration/contentcuration/utils/assessment/qti/schema/xsd/imsqti_itemv3p0p1_v1p0.xsd)

AI usage

I used Claude Code to check the QTI 3 schema's content model for gap match, compare it with TipTapEditor's current features and the changes in #6151, and draft this issue. I reviewed and edited each section.