MessagePreview learned it overflowed from onLayout, which arrives after
the mount has painted on web and Fabric. Every mount (opening the chat,
a prompt arriving, a row the list mounts again) painted the text hard-cut
with no fade or Show more, then grew the row by the control a frame later.
The expanded choice also lived in the row and reset with each remount.
Measure the body in a layout effect so the first commit carries the
control, and keep the expanded choice in a per-agent store the timeline
provides, as tool calls already do.
Mohamed Boudra
ebdf2f72cb