This bug massively improved bug 1448926 and bug 1493359 with WR enabled.

Adding a test for this in bug 1626863.

Since this bug (and the ones that came before it and after it) seem to keep causing regressions or we require further fixes to this code I made myself a summary of all the changes here to understand better and verify which bugs are in automated testing or not.

2017-09-29 bug 1395501 (Ethan Lin) Where we started storing the inherited scale in the in stacking context helper.

2018-03-27 bug 1449640 (jrmuizel) Made it so that we check for preserve3d and prespective to match what FrameLayerBuilder::ChooseScale does (which we aren't using yet)
-landed with test layout/reftests/transform-3d/preserve3d-scale.html
-in bug 1626865 (2020-04-02) I improve the test to check it against a close enough 2d ref version of the 3d test instead of just checking that it's not blank

2018-03-27 bug 1449958 (jrmuizel) use the parent scale if we didn't compute one so we don't default to 1.0 and the scale gets very large

2018-12-21 bug 1415987 (tnikkel) use FrameLayerBuilder::ChooseScale in webrender
-in bug 1626863 (2020-04-02) I add a lame test that should work for this
-bug 1495163 has a usable testcase

2019-08-01 bug 1569215 (tnikkel) pass parent scale into choose scale (regression caused by by bug 1415987)
-has test landed layout/reftests/transform/1569215-1.html
-this bug and bug 1564698 also have testcases that were fixed by this

2019-08-29 bug 1553571 (tnikkel) pass the right size from display transform into stackingcontext helper (it was zero) (caused by bug 1519444 which allowed small layers with webrender)
-has test landed layout/reftests/bugs/1553571-1.html

2020-02-20 bug 1614788 key frame effect always says its animated when its not, fixed in bug 1615156 (hiro) (bug 1415987 original regressor, got worse for unknown reason, bug 1569215 improved it back to original buggy version)
-has test landed layout/reftests/bugs/1614788-1.svg

2020-03-31 bug 1626259 (tnikkel) fixed the preserve3d check to look at self and not parent to match ChooseScale (caused by bug 1569215, but the incorrect code came from bug 1449640)
-has tests landed layout/reftests/bugs/1626259-{1,2}.html

2020-05-20 bug 1637067 (tnikkel) use Combines3DTransformWithAncestors instead of preserve3d to match ChooseScale and 3d transforms get a 1.0 scale instead of inheriting the parent scale to match ChooseScale (bug 1626259 exposed this bug but didn't cause it per-say)
-has test landed layout/reftests/transform-3d/1637067-1.html

2020-11-19 bug 1662062 (tnikkel) include the presshell resolution that is used by pinch zooming in the scale. this isn't a regression, just a case that was missed.
-has test landed gfx/tests/reftest/1662062-1.html

2021-08-17 bug 1724901 (tnikkel) include resolution that comes from ancestor documents (whether via transforms or pinch zoom) in other processes in the scale. this isn't a regression, just a case that was missed.
-has test landed gfx/tests/reftest/1724901-1.html

(In reply to Timothy Nikkel (:tnikkel) from comment #24)

2021-08-17 bug 1724901 (tnikkel) include resolution that comes from ancestor documents (whether via transforms or pinch zoom) in other processes in the scale. this isn't a regression, just a case that was missed.
-has test landed gfx/tests/reftest/1724901-1.html

Added another, better test gfx/tests/reftest/1724901-2.html (used bug 1739015 to land that)

2023-04-17 bug 1804872 (tnikkel) use the stacking context scale instead of the stacking context transform when computing the size to decode images as when the scale is being animated because the transform does not take into account the animated scale
-3 tests landed layout/reftests/bugs/1804872-*.html

2023-05-24 bug 1714763 (tnikkel) when computing the local scale use the parent scale combined with the local transform instead of the parent transform combined with the local transform. when the scale of the parent transform is increasing the parent scale will be much larger than the scale of the parent transform.
-3 tests landed layout/reftests/bugs/1714763-*.html