After every edit of a document with threaded text, each frame of a
drag included, threads::reflow copied the runs and the text object of
every frame of every thread, and then dropped the copies for each
thread whose frames the edit had not changed. With a story of 50,000
characters threaded through 8 frames with a style change at every word
(9,618 runs), moving another object took 7.8 ms in a release build,
and a frame of a drag took 7.2 ms.
reflow now copies a thread's story only when one of its frames
changed. The same move takes 0.013 ms, and the drag frame takes
0.012 ms. Threads with a changed frame re-flow as before, and the
documents and undo steps after edits are the same as before.
Docs: docs/target-app-parity.md.
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