fix(preview): keep annotation toolbar controls within the guest #170

Observed behavior

The browser annotation toolbar begins at the horizontal centre of the guest instead of being centred on it. In a narrow browser panel, controls at its right edge are clipped. The annotation can still be delivered, but this does not make the hidden tools usable.

Stable finding ANNOT-TOOLBAR-1, observed in a private Linux/Electron walk of clean source 8ac8171d095a08735670d087891cdc1ec94cf32a for #167. Independent Opus5.5 HIGH viewed and measured both frames:

• Narrow guest x464–800, centre632; toolbar left edge630–632, right-side tools clipped.
• Wide guest x1057–1600, centre1328.5; toolbar left edge1329–1334. It fits only because the guest is wider.

Private captures native-annotation-toolbar.png (2026-10-29 14:07:41 UTC) and native-annotation-wide.png (14:15:02 UTC) are retained, not publicly uploaded. Independent source comparison found PickPreload.ts and its generated styles unchanged from the #167 merge base: pre-existing baseline, not a native-core gate or a new regression from browser-sheet layering.

Reproduce and source boundary

Open an ordinary embedded page, make the browser narrow, then invoke the annotation control. Compare the toolbar's left edge with the guest centre, then repeat at a wider viewport. Its complete tool set should remain reachable.

PickPreload.ts:515–555 creates a closed shadow root, injects AnnotationStyles.generated.ts, and gives the toolbar fixed top-2.5 left-1/2 ... -translate-x-1/2. The observed geometry is consistent with the translate not applying.

Hypothesis, not a measured computed-style result: the generated transform depends on Tailwind custom-property defaults declared through @property, which may not register inside Chromium shadow trees. A repair must establish that actual computed-style mechanism; do not treat the hypothesis as the observed cause.

Desired behavior

• Annotation tools stay centred and fully reachable at the narrow and wide guest sizes.
• Annotation selection, text, delivery and dismissal keep working; current successful delivery is not a toolbar-layout pass.
• Apply any correction at the shared annotation styling boundary without changing native-session state or page styles outside the annotation host.

No source change is included. Track this separately from #167, and close only after verified repair and promotion to main.