In dom/media/gtest/TestMediaDataEncoder.cpp, the FrameSource test helper is initialized with a size through FrameSource::Init(aSize), which it stores in its mSize member. But FrameSource::GetFrame hardcodes the global kImageSize when building the frame's display size instead of using mSize:

RefPtr<MediaData> frame = VideoData::CreateFromImage(
    kImageSize, 0,
    ...

So a FrameSource created at any size other than kImageSize produces frames that report the wrong display size. For example, mData4K is initialized at kImageSize4K (3840x2160) but its frames still claim a 640x480 display size. Change kImageSize to mSize on that line so the helper reports the size it was actually initialized with.

Code: TestMediaDataEncoder.cpp:139

To verify, the encoder gtests should still pass: ./mach gtest 'MediaDataEncoderTest.*'

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https://hg.mozilla.org/mozilla-central/rev/a8d1eb56b6a1