When the desktop app opened a document on a thread of its own, that
thread parsed it, but open_read, which the app ran on the UI thread,
read the document's linked files, decoding BMP, PSD and RGB TIFF files
and storing them again as PNG, and laid out the type. In release builds,
a document with 12 linked 3000 x 2000 TIFFs kept the UI thread busy for
1.04 s, and one with 300 Auto Size area type objects, every second one
in three columns, kept it busy for 0.85 s.
read_for_open now reads the linked files and lays out the type on the
opening thread, with the path and the Update Links preference that
opening::start passes it, and open_read only adds the finished document.
open_read now takes 0.02 ms and 0.1 ms on those documents. For the
measured documents, the opened documents, results and undo states are
the same as on main.
Docs: docs/target-app-parity.md.
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