Date: 2026-03-01
Issue: #180

Bridge spike runs showed failed_by_reason={"scissor":...} while webviews intermittently appeared as black/hidden tiles, consistent with scissor-state leakage across compositor callbacks.

In shell/desktop/workbench/compositor_adapter.rs:

1. Added explicit scissor-box capture/restore helpers:
   • capture_scissor_box(...)
   • restore_scissor_box(...)
2. In run_content_callback_with_guardrails(...):
   • capture pre-callback scissor box,
   • detect post-callback scissor-box drift,
   • restore scissor box when drift is detected,
   • verify restoration outcome,
   • treat scissor-box drift as violation for replay/diagnostics reporting.
3. Updated violation emission path to include either base GL-state drift or scissor-box drift.

• cargo test -q --lib replay_channels_emit_for_sample_and_violation_artifact
• cargo test -q --lib bridge_spike_measurement_payload_contains_contract_fields
• cargo check -q

• Prevent callback-scope scissor-box leakage from clipping subsequent webview composition.
• Reduce or eliminate black-tile symptoms caused by stale scissor state.
• Keep #180 diagnostics attribution consistent with true failure source.