Fix Chromium issue 473987435: Hide masked content until all mask-image layers settle

[hjanuschka]

Chromium Issue

https://issues.chromium.org/issues/473987435

Description

Elements using CSS mask-image briefly rendered unmasked while any mask layer was still loading, causing a visible flash of unmasked content.

Fix

Hide masked content while any mask-image layer is still loading and reveal it only once every layer settles; failed loads are treated as transparent black.

CL

https://chromium-review.googlesource.com/c/chromium/src/+/7620503

Demo

https://static.januschka.com/i-473987435/

Status

Merged.