Tester feedback, relayed by Flo: the transition from the tab list to the chat
field is too hard, and the tabs could present themselves better. Three causes,
three changes, no structural touch -- tabs stay tabs, per the standing decision.
The full-width border line under every row was a ladder of hard cuts. It is a
fading rule now, starting past the icon column and dissolving before the right
edge -- the same shape the section headers have used since v1.11.0, at about
half the opacity.
The gap between the sidebar group and the message area was a bare strip of
window background with a hard edge on both sides. A faint surface wash fades
across it toward the messages, turning the cut into a seam.
And the selected tab bridges that gap: its active fill extends across the
spacing so it touches the conversation it selects -- the classic tab metaphor,
attached instead of adjacent. The bridge is a RowStyle knob (default zero), so
sidebar rows opt in and nothing else inherits it. Hover fills picked up the
standard three-pixel rounding on the way.