The scene stops heating the MacBook

"Terribly high CPU usage, the MacBook gets really hot" - a fanless MacBook
Air on a 144Hz screen. Measured on that machine with headed engines
(process CPU plus the per-process GPU time macOS keeps): an idle Aero
library cost LibreWolf ~110% CPU and ~230ms/s of GPU, the study feed ~130%,
Chromium up to ~57% CPU and ~350ms/s of GPU, Safari ~220-250ms/s of
WindowServer GPU. Priced one by one:

- the gradient mask on .ice-floats re-rendered the animated layer offscreen
  on every frame (~75 points of Gecko CPU). The floats now rise through a
  plain rectangular band clear of the header, the bottom chrome and the
  rail, and fade in and out inside it, so the clip never cuts a visible one;
- the overlay grain forced a backdrop readback per frame for a texture that
  measured a mean <=0.11/255 difference: removed;
- the invisible blob drift made every frame a full-screen redraw: the blobs
  are painted once;
- static glass (answer tiles, feedback panel, chips, inputs, dropzones,
  lists, segmented controls, Aero hint chips) re-blurred the moving scene
  every frame: only floating glass blurs now, and Aero's baked chat chrome
  drops a live blur that never showed under the bake;
- the scene also parks while the window has no focus and under any dialog
  or sheet scrim (.scene-pause); the loading icon is a compositor-run CSS
  animation instead of a framer-motion main-thread loop.

After, focused and animating: LibreWolf library 109% -> 33% CPU, study
134% -> 34%, chat 130% -> 42%; Chromium GPU 2.5-4.4x lower with 7-22 points
less CPU; Safari's WindowServer GPU -20-26% in Aero, about half in dark.
Unfocused or under a sheet: nothing. bubbles-test sweeps every float through
41 phases against the band and pins the parking and the cost contract;
tests/heat-probe.mjs (macOS) repeats the measurement; the suites find
Playwright's Chromium on macOS.

All 15 suites pass, lint clean, the contrast scan lost a finding. Version
0.6.7, SW v89.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -217,16 +217,18 @@ the saved notes) with the matching words highlighted green, and the feed chrome'
   their own bottom action bar instead of the tab bar, and the in-page segmented switch moves between
   the deck and homework wizards.
 - **Frosted ice glass, both themes**: light page `#eef5fd` (pale ice) with white glass panels;
-  dark page `#0a1120` with `rgba(255,255,255,.06)` glass. Every large surface shares one recipe —
-  translucent tint, 24px `backdrop-blur` (16px on phones), a bright 1px rim, an inner top highlight
-  and a blue-tinted shadow — over a fixed ice scene (navy/pale base, drifting glacier blobs, glossy
-  ice orbs and a whisper of grain).
+  dark page `#0a1120` with `rgba(255,255,255,.06)` glass. Every surface shares one recipe —
+  translucent tint, a bright 1px rim, an inner top highlight and a blue-tinted shadow, plus a real
+  16px `backdrop-blur` (12px on phones) only on floating glass that content scrolls under — over a
+  fixed ice scene (navy/pale base, soft glacier blobs and glossy ice orbs rising through the open
+  band between the chrome).
   Text tiers are documented AA ratios (see below), and the cards fill the row with
   `repeat(auto-fill, minmax(280px, 1fr))` — no stranded cards, no dead space
 - Deck cards: title → muted metadata → neutral tags (difficulty carries the only colour:
   green/amber/red) → summary → a single **Study** button, with everything else in the `⋯` menu
-- **Frosted UI**: the sticky header, tab bar, sheets and panels use real `backdrop-filter` blur over
-  the fixed ice scene (drifting glacier blobs, glossy orbs, a whisper of grain) — depth without noise
+- **Frosted UI**: the sticky header, tab bar, study chrome and sheets use real `backdrop-filter`
+  blur over the fixed ice scene — depth without noise. Glass that sits still never blurs, and the
+  scene parks whenever nobody is watching it (hidden tab, unfocused window, an open dialog)
 - **Pill language**: primary CTAs are near-black in light mode and white in dark mode; secondaries
   are outlined or ghost. Colour is reserved for meaning — activity tints (sky/lilac/mint/peach/rose),
   correct/wrong feedback, key status
@@ -1434,6 +1436,53 @@ Things I chose while building, with the reasoning:
     (measured: 268..1280 at a 1280 window) — a flat pane floating 40px inside the scene read as
     "cut off next to the sidebar".
 
+98. **The MacBook ran hot on the scene, so the scene's per-frame bill came down.** "It has
+    terribly high CPU usage and the MacBook gets really hot when I'm on it" — a fanless MacBook Air
+    driving a 144Hz display. Measured on that machine with real headed engines (process CPU plus
+    the per-process GPU time macOS keeps — `tests/heat-probe.mjs`), not frame times in a throttled
+    software renderer: an idle Aero library cost LibreWolf (Gecko) ~110% CPU and ~230 ms of GPU
+    per second, the study feed ~130% / ~320 ms, Chromium ~57% CPU and up to ~350 ms/s of GPU, and
+    Safari ~220-250 ms/s of WindowServer GPU above the desktop. Each suspect was switched off on its
+    own to price it:
+    - **The gradient mask on `.ice-floats`** (from 91) re-rendered the whole animated layer
+      offscreen on every frame — ~75 points of Gecko CPU by itself. The floats now rise through a
+      plain rectangular band (68px below the top, 151px above the bottom — 172px on phones — and
+      right of the 268px rail strip on desktop), and every orb and bubble starts at the band's
+      bottom and ends at its top at zero opacity: the fades live in the keyframes (in by 18%, out
+      from 90%; `--ice-rise` is the band height minus the float's own) and the 1.06 end scale is
+      taken back by a 3% nudge, so no visible float ever touches the clip. The bubbles suite sweeps
+      every float through 41 phases at 1280 and 390 and fails on any visible frame outside the band.
+    - **The overlay grain** (`mix-blend-mode` over the animated layer) forced a backdrop readback
+      per frame for a texture that measured a max 2-6/255 and mean ≤0.11/255 difference —
+      invisible. Gone.
+    - **The blob drift**: four viewport-sized layers moving a few vw over 44-60s at 0.045-0.1
+      opacity turned every frame into a full-screen redraw (Chromium GPU 128 → 80 ms/s with them
+      still). They are painted once now.
+    - **Backdrop blurs on glass that sits still**: answer tiles, the feedback panel, chips, inputs,
+      dropzones, grouped lists, segmented controls and Aero's hint chips re-blurred the moving
+      scene every frame (the study feed's tiles alone: ~20% Chromium CPU, ~270 ms/s of GPU). Only
+      floating glass that content scrolls under blurs now; the hint chips use the shared bake, and
+      the baked Aero chat header/composer drop a live blur that never showed under the opaque bake
+      (touch has no bake there and keeps it). Their bake is the glass on desktop, so 97's flat
+      nested tiles still sit on the right backdrop.
+    - **Nobody watching**: besides the hidden tab, the scene now parks while the window has no
+      focus (another app in front, the browser on the other screen) and while any dialog or sheet
+      is open — its scrim carries `.scene-pause`, so the blurred scrim and sheet render once.
+    - The loading screen's rocking icon was a framer-motion loop writing the transform from the
+      main thread every frame for the whole generation; it is a compositor-run CSS animation on
+      `rotate`/`scale` now.
+
+    Same machine, focused and animating (before → after): LibreWolf library Aero 109% → 33% CPU
+    (GPU 227 → 101 ms/s), study 134% → 34% (316 → 141), chat 130% → 42%, dark/light library ~94% →
+    24-26%; Chromium's GPU time 2.5-4.4× lower (study Aero 354 → 80 ms/s) with 7-22 points less CPU;
+    Safari's WindowServer GPU above the desktop −20-26% in Aero and about half in dark. Unfocused,
+    or with a sheet open, the scene costs nothing. What remains is the price of the ambient motion
+    itself — the orbs (23 floats in Aero) composited at the display's refresh rate, on the
+    compositor only (Chromium's main thread: ~9ms busy per 4s); Settings → Reduce motion parks it
+    entirely. The visuals are unchanged apart from where a bubble happens to be at a given moment,
+    and the pixel-contrast scan lost a finding (the Aero study chip, 4.49:1). The suites also find
+    Playwright's Chromium on macOS now (`~/Library/Caches/ms-playwright`).
+
 97. **Nested tiles inside the real-blur chrome drop the baked backdrop.** "Frutiger aero theme —
     the attachment button looks completely different to the other ones." The composer band, the
     chat header and the study pager keep a real `backdrop-filter` (they float over scrolling