fix(shim): live viewport size and a ResizeObserver that fires

[pathscale]

Two web-API shim bugs in apps/chuzz/src/document_loader.rs, found while debugging an @pathscale/ui Select on crates.vip. The dropdown opened at the left edge of the window and stayed there. Both stubs were written when nothing could measure a box from script; getBoundingClientRect now returns laid-out boxes, so both can be real.

1. innerWidth / innerHeight reported a fixed 1440x960 forever

The engine (ps-blitz-script 0.4.11) owns them as live accessors over the viewport. A page's scripts run before the host sets that viewport, so at boot they read 0. The shim took 0 for "missing" and replaced the accessors with 1440x960 for good.

Fix: capture the engine's getters and answer with them, falling back to 1440x960 only while they still read 0. screen and matchMedia's dimension branch read the same live size. Assigning innerWidth keeps the assigned value (replaceable attribute). No getter reads a shimmed name, so the old recursion cannot return.

2. ResizeObserver never fired

A silent stub. A component that measures, moves and relies on the observer to settle was left wherever its first frame put it.

Fix: a real observer over border boxes, measured on animation frames. With no engine resize notification, measuring is armed by observe(), resize/scroll, pointer, keyboard and input events, and by a size change it finds; it disarms after 30 quiet frames, so a page that observes something permanently still goes idle for rendered QA's idle and drift checks. Entries follow the specification's shape; contentRect and the box sizes describe the border box because computed style does not report padding to script.

Measured (chuzz-headless, 1344x900, a Select in a table row, stock @pathscale/ui 3.2.4)

	before	after
innerWidth x innerHeight after the viewport is set	1440x960	1344x900
ResizeObserver on a table, then resized	nothing	0 to 968, then 968 to 600
Select list vs its trigger	left 88 under a trigger at 512, never corrected	first frame at 8, then 479 under its trigger at 479

The remaining one-frame misplacement is @pathscale/ui's own, fixed in pathscale/ui#296.

Checks

• cargo build --release -p chuzz --bin chuzz-headless --no-default-features --features capture,javascript: no warnings.
• cargo test --release -p chuzz --no-default-features --features capture,javascript: 81 pass, 3 fail. The 3 are capture::tests that need system-fonts (a blank capture without a font); they fail identically on master with the same features.
• cargo fmt -p chuzz -- --check: clean.
• Not run: a build with system-fonts, the GUI.

3. ResizeObserver reports the content box

Follow-up now that ps-blitz reports padding and border (pathscale/ps-blitz#102): the content box is the border box less both, which is what an observer watches by default; { box: 'border-box' } is honoured. On an engine without the edges they read as zero and the content box falls back to the border box (the previous behaviour), so this is safe on the published engine.

Measured on a box with content 100x20, padding 10, border 2: published engine 124x44 (fallback); ps-blitz#102 content 100x20 at x 12, border box 124x44.

Fixed in the engine instead (pathscale/ps-blitz#102)

• MutationObserver: the shim's stub never fired. ps-blitz now defines a real one; the shim's typeof ... === 'undefined' guard then leaves it alone. chuzz picks it up with the next ps-blitz release (caret ^0.4).
• getComputedStyle padding, border and margin, and clientWidth as the padding box.

Release

Bumps chuzz to 0.1.40, so merging publishes a new headless-v0.1.40 host. Fleet sites whose QA pins the headless-host action need their pin moved to pick it up.

Against the published engine

ps-blitz 0.4.12 is on crates.io and this branch resolves it through ^0.4 with no manifest change. A release build from crates.io (no local override) on the probe page: MutationObserver reports attribute and child changes, ResizeObserver reports a 100x20 content box (border box 124x44), innerWidth is 1344, and the Select opens under its trigger.

Hard-coded local path

scripts/qa-visible-app.sh made its default artifact directory under /private/tmp, which exists only on macOS; it now uses ${TMPDIR:-/tmp}, where the rest of the script already looks. bash -n passes and the default resolves under TMPDIR.