With Type › Show Hidden Characters on, the canvas laid out every
visible text object in view in every frame to place the dots on
spaces and the ¶ and # marks. With 1,000 two-line labels in view,
drawing the marks took a median of 19.7 ms per frame in a release
build.
The overlay now keeps each text object's marks in document space,
together with the node it laid them out from. In a frame it lays out
a text object only when its node is a new `Arc` (an edit makes one)
or the frame before did not draw it, and it lays out all type in view
again when the fonts or the hyphenation exceptions change. The marks
are still placed for the current view in every frame. With the same
labels, drawing the marks takes 0.8 ms per frame, and the shapes drawn
are the same as before.
Docs: docs/target-app-parity.md.
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