Remove dead vertical-tab separator and sidebar traffic-light paths from
the unified chrome layout. Keep the toolbar stack owned by TabWidget so
layout changes no longer move each tab toolbar in and out of its page.
Share the vertical New Tab button setup between initial layout and
hover expansion, and keep window-control sizing helpers private.
Treat the toolbar, tab strip, and vertical sidebar as one browser
chrome surface. Add an explicit layout policy for platform window
controls and sidebar widths. This lets macOS use left-side traffic
lights while other platforms keep right-side custom controls.
Keep the macOS toolbar stable in vertical-tabs mode. Traffic lights and
navigation controls stay in the top row, and the sidebar sits below it.
Draw chrome/content separators explicitly so the content edge remains
clear without adding seams inside the chrome surface.
Retune tab, toolbar, and omnibox styling for the unified chrome. Tabs
use card-like selected states, quiet inactive states, and matching
horizontal and vertical geometry.
Make hover-expanded vertical tabs draw as a chrome overlay so the web
content does not relayout. Keep the overlay above native web views and
route macOS window drags through AppKit events for reliable dragging
from custom chrome.
Add a shared WindowControlButton for the frameless window controls. It
clears the pressed visual state when the pointer leaves during a press,
and restores it on re-entry.
Also reset transient button state around double-clicks so pressed
styling cannot get stuck after the window state changes.
