layout.rs (headless, tested): Placements remembers each tab webview's last rectangle and
visibility, so a tabs_sync that changes nothing makes no native call and one that moves one
pane touches one webview; Frame/follow_rect estimate where panes go when the window size
changes (outer margins fixed, proportional in between, shared borders stay shared).
host.rs: place_tab/forget_tab use it; place_child is a single set_bounds (one WebView2
SetBounds instead of a move then a resize, each a page re-layout); follow_window_resizes
re-places visible tabs from the window's own Resized events on the main thread, before the
shell lays out again, and keeps WindowMetrics current so a sync needs no blocking round
trips for the window size; layout_window says when a layout was made for a stale size.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
