Bugzilla

(In reply to Daniel Holbert [:dholbert] from comment #12)
> I'm not sure yet what caused us to go from blank-SVG-area to line-partially-drawn-in-SVG-area, but I'm willing to bet it was another graphics backend change (perhaps a Skia upgrade).

Pushlog for this change (with non-blank partial-line-drawn SVG area starting at Nightly 2017-04-07):
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=3c68d659c2b715f811708f043a1e7169d77be2ba&tochange=10ea10d9993c9701e5525928257a589dea2c05d8

I'm not sure what changed in there to get things less-broken; perhaps it was one of the SVG transform-related commits.

In any case, though: it looks like we can still force the cairo drawing backend even in current Nightly builds -- and with that forced, I get EXPECTED RESULTS (full vertical line) from the original testcase. Here's the command I'm using (notice 2020-06-01, i.e. yesterday's Nightly):
```
mozregression --launch 2020-06-01  -a https://bugzilla.mozilla.org/attachment.cgi?id=9058252  --pref "gfx.content.azure.backends:'cairo'"
```

So, this strongly suggests that the testcases here are symptoms of a precision bug in our Skia and WebRender graphics backends (likely due to 32-bit floats losing precision at large values, or something along those lines.)

--> Reclassifying as Graphics

(In reply to Daniel Holbert [:dholbert] from comment #12)
> I'm not sure yet what caused us to go from blank-SVG-area to line-partially-drawn-in-SVG-area, but I'm willing to bet it was another graphics backend change (perhaps a Skia upgrade).

Pushlog for this change (the partial restoration of functionality, bringing us to the current behavior, with a partial-line drawn in the SVG area starting at Nightly 2017-04-07):
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=3c68d659c2b715f811708f043a1e7169d77be2ba&tochange=10ea10d9993c9701e5525928257a589dea2c05d8

I'm not sure what changed in there to get things less-broken; perhaps it was one of the SVG transform-related commits.

In any case, though: it looks like we can still force the cairo drawing backend even in current Nightly builds -- and with that forced, I get EXPECTED RESULTS (full vertical line) from the original testcase. Here's the command I'm using (notice 2020-06-01, i.e. yesterday's Nightly):
```
mozregression --launch 2020-06-01  -a https://bugzilla.mozilla.org/attachment.cgi?id=9058252  --pref "gfx.content.azure.backends:'cairo'"
```

So, this strongly suggests that the testcases here are symptoms of a precision bug in our Skia and WebRender graphics backends (likely due to 32-bit floats losing precision at large values, or something along those lines.)

--> Reclassifying as Graphics