I've found another way to solve this bug; possibly a better way. Full-page zoom is triggering an unnecessary resolution change in ShrinkDisplaySizeIfNeeded->UpdateResolution due to aViewportInfo.IsDefaultZoomValid() being true at https://searchfox.org/mozilla-central/source/layout/base/MobileViewportManager.cpp#389. IsDefaultZoomValid() is true because initial-scale is specified. But that shouldn't make content-fitting behave differently. So checking IsDefaultZoomValid() here is probably incorrect and we need something more specific for the intended cases.
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I've found another way to solve this bug; possibly a better way. Full-page zoom is triggering an unnecessary resolution change in ShrinkDisplaySizeIfNeeded->UpdateResolution due to aViewportInfo.IsDefaultZoomValid() being true at https://searchfox.org/mozilla-central/source/layout/base/MobileViewportManager.cpp#389. IsDefaultZoomValid() is true because initial-scale is specified. But that shouldn't make content-fitting behave differently after the initial display of the content. So checking IsDefaultZoomValid() here is probably incorrect and we need something more specific for the intended cases.