A 6px handle on the sidebar's trailing edge (col-resize cursor, accent
tint on hover) sets the OverlaySplitView width, clamped to 200–560px.
The drag gesture sits on the sidebar overlay, whose leading edge stays
fixed, so the pointer x is the width and it tracks the cursor exactly.
The width persists in gui.json as `sidebar_width`; absent, the old
260–340px fraction default applies.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>