Bug 1668913 Comment 1 Edit History

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Original comment by
on 
(In reply to Botond Ballo [:botond] from comment #0)
> Since then, we've realized that in some cases where we thought the scales will be the same, they can in fact differ.

In particular, without WebRender, `FrameMetrics::mCumulativeResolution` can include the scale component of a CSS transform applied to the scrolled content, and that CSS transform can have different x- and y-scales.

So, usages would fall into "the two scales can be different" case if:

  * the `ScaleFactors2D` in question is `FrameMetrics::mCumulativeResolution`, or derived from `FrameMetrics::mCumulativeResolution` without dividing out the portion that includes the x- and y-scales; **and**
  * the codepath is not specific to WebRender.
Revision 1 by
on 
(In reply to Botond Ballo [:botond] from comment #0)
> Since then, we've realized that in some cases where we thought the scales will be the same, they can in fact differ.

In particular, without WebRender, `FrameMetrics::mCumulativeResolution` can include the scale component of a CSS transform applied to the scrolled content, and that CSS transform can have different x- and y-scales.

So, usages would fall into "the two scales can be different" case if:

  * the `ScaleFactors2D` in question is `FrameMetrics::mCumulativeResolution`, or derived from `FrameMetrics::mCumulativeResolution` without dividing out the portion that includes the CSS transform scales; **and**
  * the codepath is not specific to WebRender.

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