This bug is due to the high-quality image downscaler that was introduced in bug 486918. The normal downscaler and the high-quality one round their target dimensions differently, e.g. take a 1920x1200 image with max-width:1085px, which ought to be scaled down to 678.125px tall. The high-quality resizer instead resizes it to 679px tall and then crops the bottom pixel off. The normal downscaler resizes it to 678.125px tall and then crops off the bottom 0.125px. Since images are first rendered with the normal downscaler, and then replaced with the high-quality version, you see a visible shift in images when the high-quality algorithm kicks in. I first noticed this by opening a bunch of large images with different dimensions as top-level images in a bunch of tabs, waiting the 10s until the images in the background tabs were discarded, and then switching between the tabs. Depending on the exact dimensions of each image, some of them shift when they're decoded again. I have a slightly less psychedelic example page here: http://tinyurl.com/k4pzug3. Click the button repeatedly, then try again with image.high_quality_downscaling.enabled = false.
Component: Untriaged → ImageLib
Product: Firefox → Core
I think that's a dupe of bug 925611.
My example page no longer shows the shifting, so I'd say this is fixed.
Status: UNCONFIRMED → RESOLVED
Closed: 5 years ago
Resolution: --- → FIXED
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