A black bar in ffmpeg screen recordings traced to Chrome's window frame
Claude Code explained a 37-pixel black bar in HTML-to-MP4 recordings as Chrome's window frame offset and fixed the crop with xprop.
Credits
- Reddit poster According_Turnip5206 Source ↗
Earliest dated source:
The full story
The poster was exporting HTML canvas animations to MP4 by serving the page, opening Chrome in app mode and recording the screen region with ffmpeg and x11grab. The recordings had about 37 pixels of black at the top, and manual crop tweaking did not help.
Claude said Chrome's --window-size flag sets the outer dimensions including the title bar and borders. It ran xprop _NET_FRAME_EXTENTS on the Chrome window to get the exact edge values and used them to compute the correct content offset. The fix was about four lines in the script.
How it was done
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Sources
From a short post by the maker, so the details are as they tell them.
- Reddit: Asked Claude to record HTML animations to MP4. It debugged a Chrome quirk I didn't even know existedReddit poster: According_Turnip5206Posted
The maker's post on r/ClaudeCode.
If it disappears: archived copy of Reddit: Asked Claude to record HTML animations to MP4. It debugged a Chrome quirk I didn't even know existed (opens in a new tab)