As an optimization, FontCascade may render some glyphs to a DisplayList and then replay that list instead of rendering again. But when replaying, the glyphs are not rendered through a GraphicsContext as we would expect, but using FontCascade directly. In the cairo case this means that FontCascade requires a cairo context to render, but if we are in the recording phase of the rendering there's no platform context and we have the crash.
Created attachment 343231 [details]
Created attachment 343313 [details]
Comment on attachment 343313 [details]
Clearing flags on attachment: 343313
Committed r233075: <https://trac.webkit.org/changeset/233075>
All reviewed patches have been landed. Closing bug.