Native surface coverage

Preserve con's glass material while keeping each moving pixel covered by its correct owner. A darker result, passing layout test or clean log is insufficient.

Identify the failure before choosing a fix

Record direction and trigger, tab orientation, compact/pinned/hover state, pane/surface layout, input-bar visibility, theme, terminal/UI opacity, blur, macOS version, display scale and release versus preview binary.

Observation	Investigate
Different backdrop colors visible through adjacent surfaces at rest	Intended transparency; compare the same position and backdrop
A stable hairline or jagged curve	Physical-pixel endpoints, shared corner coverage, overlapping alpha
A broad moving strip, especially one that also clips text	Native frame/mask/GPUI presentation timing; measure successive video frames
Correct model bounds but wrong native output after reuse	Attached masks, native backing-layer replacement and actual framebuffer dimensions

Remote desktop or OS differences are verification limits, not presumed causes. For a transient report, inspect a recording before treating a helper-level bug as the root cause. A strip advancing with per-frame travel is evidence of a presentation mismatch, not a reason to thicken its edge.

Trace the real rendering path

Read the relevant sections, not every historical investigation:

• Workspace composition and frame geometry.
• Pane ownership, native host and AppKit masks.
• Native presentation contract.
• For cache/opacity changes, GPUI cache guidance.