perf(glass): pause the lens while the page scrolls, with a setting to keep it

[Zakkaus]

Liquid Glass drops frames whenever the page scrolls. Chromium redraws every lensed backdrop (cards, sidebar, bottom bar, floating panel) on the CPU each frame the page moves under it.

While the window scrolls, and for 150 ms after, src/ui/lens.ts marks the root with data-lens-paused, and the lens tokens fall back to the same blur and colour without the refraction. At rest the look is unchanged; the bent edge and rim return 150 ms after scrolling stops.

Measured in headless Chrome on an RTX 4080 SUPER with hardware compositing, 4 s of continuous scroll, median of 3:

Page	Before	After	Clear (no lens)
Desktop activity	41.8 fps, 11.1 ms draw	60 fps, 0.30 ms	60 fps, 0.22 ms
Desktop connections	46.4 fps, 10.2 ms	60 fps, 0.17 ms	60 fps, 0.12 ms
Phone activity	60 fps, 3.0 ms	60 fps, 0.19 ms	60 fps, 0.15 ms

"Draw" is the renderer compositor's MainFrame.Draw time per frame. Removing only url(#doona-lens) restored the baseline; removing the leading blur changed nothing.

Not addressed here: at rest, live updates still redraw the lens, at about 4.8 ms a frame on the desktop activity page against 1.3 ms without it.

Settings > Appearance has a switch for the pause, on by default and shown only with Liquid Glass. Turned off, the root carries data-lens-steady and the lens keeps refracting while scrolling. The Glass rows (wallpaper, blur, this switch) now load lazily with the Glass palettes, which keeps the Settings route in budget: 67,133 bytes gzip against a 72,800 limit, down from 72,757.

The Activity startup budget rises from 341,850 to 342,050 bytes gzip: the setting adds 151 bytes to that path (the preference, its effect and two catalogue strings), and CI measured 341,883.