There is a number of minor regressions and a serious accessibility issue reported against inspector after the migration to the canvas-based editor. We should bring the original div-based editor back, make SourceFrame pluggable and hide canvas-based editor behind a flag for now. In the meanwhile, we could fix the regressions and work on our accessibility story.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.