Bug 208161 – [Frame flattening] When the iframe content schedules layout, it should also mark the owner renderer dirty

• Status: NEW
• Component: Layout and Rendering

[alan]

In the frame flattening context when optional layout is delayed, getComputedStyle() might return the un-flattened geometry unless the iframe element is forced to get laid out.
        
1. main frame content is loaded and laid out -> if the iframe content is not ready yet, we don't initiate iframe flattening.
2. iframe content is ready -> layout is scheduled.
3. getComputedStyle() is called which initiates a style update/layout on the main frame, but only dirty boxes trigger layout (the iframe renderer itself is not dirty <- this is where frame flattening fails)
4. getComputedStyle() returns with the "stale" geometry.

See fast/frames/flattening/iframe-tiny.html

[Radar WebKit Bug Importer]

<rdar://problem/59744384>