Page.startScreencast frames are scaled to fit maxWidth/maxHeight, but the
frame metadata's deviceWidth/deviceHeight keep the pre-scale device size.
On viewports wider than the 1280px cap the crop was computed in the wrong
coordinate space: shifted toward the bottom-right and, for a cursor there,
hanging off the actual image — drawImage clipped it and the video showed a
squashed corner plus letterbox. Decode the first frame to size the output
and crop each frame by its real bitmap dimensions instead.

Co-Authored-By: Claude Code <noreply@anthropic.com>