The canvas only showed effects after OK. While the dialog is open, the
canvas now renders the document with the dialog's style applied: the same
commands OK runs, replayed on a scratch session (no history), cached by
document revision + style fields so switching pages doesn't re-render.
Cancel drops the preview. Works on both GPU and CPU canvas paths
(display_doc). Tests cover the preview, re-render on edit, and cancel.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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