fix(layout): guard the default tab-stop grid against a non-positive interval

[leonidkuznetsov18]

What

A DOCX whose word/settings.xml carries <w:defaultTabStop w:val="0"/> froze the main thread forever: onReady/onException never fired and the tab pegged the CPU until killed (#3944).

computeTabStops (packages/layout-engine/contracts/src/engines/tabs.ts) generates the automatic tab-stop grid with pos += defaultTabInterval toward a fixed limit, so an interval of 0 (or a negative one) never advances and the loop never terminates. Both buildTabStopsPx callers (measuring/dom and layout-bridge/remeasure) pass the document value through tabIntervalTwips ?? DEFAULT_TAB_INTERVAL_TWIPS, and ?? doesn't catch 0. The grid is pre-computed for every measured paragraph, which is why any document with text content triggers it — no tab characters required (an empty <w:p/> escapes via the simple-empty-paragraph fast path, matching the repro matrix in the issue).

Fix

Skip the default grid for a non-positive (or non-finite) interval, keeping explicit stops and the implicit indent-driven stops intact. Word writes defaultTabStop 0 when a user sets the default tab interval to 0, opens such documents fine, and simply generates no automatic tab stops — so "no grid" is the Word-faithful behavior, and it mirrors the existing guard in word-layout's own computeTabStops (packages/word-layout/src/tab-layout.ts).

Guarding this single choke point covers both callers; it is the only += interval loop over the document value in the layout engine.

Testing

• New regression tests in tabs.test.ts for interval 0 (no default grid, terminates) and a negative interval (explicit stops preserved, no default grid). Without the guard the first test hangs the runner — verified by stashing the fix: the suite ran into a 3-minute wall timeout exactly as described in the issue.
• packages/layout-engine/contracts: 492/492 pass.
• Downstream consumers unchanged for positive intervals: measuring/dom 529/529, layout-bridge 1653/1653 pass.

Fixes #3944

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[github-actions]

All contributors have signed the CLA ✍️ ✅
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[cubic-dev-ai]

No issues found across 2 files

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[leonidkuznetsov18]

I have read the CLA Document and I hereby sign the CLA

[caio-pizzol]

Thanks, Leonid, for the clear investigation and regression tests. The zero default tab-stop hang has been addressed in V2 and shipped in 2.13.0. We also verified that the zero-value fixture opens and remains responsive in 2.14.0. The shipped fix falls back to the standard tab interval, so we’re closing this PR as superseded for the reported hang. Please upgrade to 2.13.0 or later.