Let style changes that only rebuild compatible accumulated visual
contexts avoid marking the display list dirty. This lets transform
and nonzero opacity updates send visual context tree updates without
recording a new display list.
Keep repainting changes that affect display-list contents or can change
visual context tree compatibility, including zero-crossing opacity,
transform invertibility crossings, background-attachment, clipping,
mix-blend-mode, and perspective. Schedule accumulated visual context
updates for animations independently of repaint so animated
transform/effect updates keep reaching the document.
Cover compatible visual context reuse, incompatible tree shapes, and the
display-list invalidation cases with focused LibWeb tests.
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