STR:

1. At least on macOS, with webrender enabled, visit https://www.roomformilly.com/

Expected: The letter R in Room is visible
Actual: It's invisible

If I disable the mask that's a sibling to the path that strokes the R (<mask id="4c46b4b5-2005-4e0f-bd0d-2bb4293a8c3f" height="72.4" width="51" y=".8" x="0" maskUnits="userSpaceOnUse">), the bug disappears.

Jessie, are there known WR bugs with SVG masks, perhaps?

This got progressively worse with more of the R getting not drawn over time.

R -> top half of R only
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=f4e9e5429fd82f9c3787ca2d42798a2bf3d353f9&tochange=20d6f233678b8cbbfec09f85c40ae42e21263a72

top half of R only -> top half of R only and clipped on the right
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=11dc1c09bb580dfec90d93bfdf2fae386f670d0e&tochange=ba22d4def27fcbfe82f102f9562fd25eb1b4f7b2

top half of R only and clipped on the right -> no R at all
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=4fe07886a969323cee79798ec501f43882096a61&tochange=5fde2c3df6087606aed6f906ee2e8d679b3e4493

I suspect this is caused by us not respect the DataSourceSurface() offset in the composite filter

By using the destination DT we will use the correct offset
during playback instead of the offset of the reference target.

Comment on attachment 9136204 [details]
Bug 1617708. Make ClippedDrawTarget use destination DT.

Beta/Release Uplift Approval Request

• User impact if declined: Some web content that uses svg masks inside of filters will render incorrectly. e.g. https://roomformilly.com
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): The previous code didn't make much sense. The change is pretty mechanical and it's hard to imagine a scenario where it would be less correct.
• String changes made/needed:

https://hg.mozilla.org/mozilla-central/rev/0e08a1d7fb07

Comment on attachment 9136204 [details]
Bug 1617708. Make ClippedDrawTarget use destination DT.

approved for 75 rc1

https://hg.mozilla.org/releases/mozilla-release/rev/7a72ca975023

https://hg.mozilla.org/mozilla-central/rev/ee6e2b8bea74

Reproduced the initial issue using Fx 74.0 on macOS 10.15.4, verified that the issue is no longer present using latest Nightly 76.0a1 and Fx 75.0 RC on the same platform.