In bug 1140619 we are trying to fix issues that arise from the interaction between decode-on-draw-only, downscale-during-decode, and the B2G Gallery app. The Gallery app has a view where it displays images in a carousel. The current image is displayed inside the viewport. Just to the left of the viewport is the previous image in the carousel, and just to the right is the next image. We want to regard those images as visible, locked, and decoded, even though they're in the overflow area and not actually visible. It should be possible to do this by expanding the visible rect we compute here by a few CSS pixels: https://dxr.mozilla.org/mozilla-central/source/layout/generic/nsGfxScrollFrame.cpp#2769
Summary: Treat content just outside a scrollframe as visible even if we can't scroll to it → Treat images just outside a scrollframe as visible even if we can't scroll to it
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