When image data becomes available, intrinsic dimensions can change, but
that does not require relayout for image elements whose computed width,
height, and min/max constraints are independent of intrinsic sizing. In
that case, invalidate paint and intrinsic-size caches instead of marking
layout dirty.
Keep the conservative relayout path for auto, percentage,
calc-percentage, and intrinsic sizing constraints, since decoded image
dimensions can affect used size in indefinite contexts. Query image
availability while painting so an existing paintable can switch from alt
text to image content without relayout.
Add text coverage that waits for image load events and checks auto,
percentage, calc-percentage, intrinsic keyword, min/max percentage, and
fixed-size cases without relying on timeouts.
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