Bug Fixes
Fixes a regression from #2023 (shipped in v1.16.0), which set color-scheme on the iframe element to fix a Firefox dark-mode bug. The document inside the iframe got no color-scheme until its bundle ran, and a mismatch between the two makes the embedded canvas opaque instead of transparent, so it painted white. Underneath that, browsers paint a default surface for an iframe before its document is parsed at all, which WebKit renders white and Chromium hides behind paint holding.
The document now sets its color-scheme in an inline head script before first paint, and the iframe stays hidden until its document reports itself ready, with a timeout fallback.