The smallest example that proves why ui.selection.capture() exists. Single file, no framework.
A comment composer cannot read the live selection at submit time. The textarea takes focus when the composer opens, the editor's live selection visually clears, and a ui.selection.getSnapshot() call at submit returns null. The fix is ui.selection.capture() at composer-open: a frozen snapshot of the selection that survives focus changes. Pass it to ui.comments.createFromCapture(capture, { text }) at submit, and the new comment anchors against the original selection.
This example shows that flow and nothing else. No threading, no resolve / reopen / reply, no toolbar, no mode toggle. For the full Custom UI sidebar pattern, see demos/custom-ui.
pnpm install
pnpm devSelect text in the document. Click Add comment on selection. Type, then post. The new comment appears in the right-hand list anchored to the original selection, even though the selection visually cleared the moment the textarea took focus.