[ragojose]

Summary

• add opaque black backing to tab containers to prevent wallpaper bleed while CEF paints
• dissolve a live AppKit glass veil above CEF in both page-to-blank and blank-to-page directions
• preserve background throttling, rapid-transition cancellation, occlusion handling, and Reduce Motion behavior
• use matched easing and a one-frame glass handoff to eliminate hard cuts and black flashes

Testing

• cmake --build build --parallel
• manually verified blank tab → loaded page
• manually verified loaded page → blank tab
• manually checked the final glass handoff for the prior black-frame jump