feat(suggestion-menu): anchor to the host's text

[kianbazza]

Summary

SuggestionMenu anchors the popup to the host's text.

• anchor: () => DOMRect | null. The popup follows the text when the page or field scrolls. A new position is computed only when the measured rect changes. null keeps the last position, and nothing is measured on close, when the text may already be gone.
• SuggestionMenu.Positioner places the popup below, aligned to the start. If there's no room below it opens above, and it keeps that side until it closes, so it never flips while the user types or results stream in. It never goes beside the anchor, where it would cover the text.

open near the bottom -> Base UI flips it above -> held above
type "q" (1 row, would fit below) -> stays above

The side is read once Base UI has positioned against an anchor measured during this opening. In a background tab, where frames don't run, it's read on the first keystroke instead.

Evidence

suggestion-menu.anchor.test.tsx (24 tests) gives the viewport a real size in jsdom:

• Flips at open, including under StrictMode.
• The side is held through typing and through an anchor move that would fit below.
• The popup never goes beside the anchor.
• Holding waits for a real anchor, including one that arrives with a keystroke or measures late on a reopen.
• The anchor is measured again on each update, not on unrelated renders or on close.

Checked in Chromium with ProseMirror at 900×290: the popup opened above, stayed above after the list shrank to one row, and moved with the text when the page scrolled.

Merge Danger

Door: two-way

Blast Radius: SuggestionMenu only

Closes UI-563

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
ui-canary	Ready	Preview	Sep 29, 2026 2:31pm UTC

[linear-code]

UI-563