User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:71.0) Gecko/20100101 Firefox/71.0
Build ID: 20190912094122

Steps to reproduce

1. In a new profile, reduce the browser window height. It seems to be easier to reproduce if more scrollings are involved.
2. Go to MathML Torture Test.
3. Scroll to the bottom then scroll up back to the main table (with numbered rows).
4. Check for defects.

Actual result

Please refer to the attached screenshot. Some symbols are stretched incorrectly. Some are misplaced or rendered incompletely.

Expected result

All symbols display correctly.

Workaround

1. Disable WebRender. Or
2. highlighted affected symbols and deselect them.

Notes

1. So far I see such issue only in MathML symbols.
2. Font choice should be unrelated. I tried some fonts (Latin Modern Math, DejaVu, XITS) and they all corrupt.

System info

Windows 10 1903 build 18362.356 (latest with all updates installed as of 2019-09-13)
Nvidia GTX 760 driver 436.30 (latest)
Latin Modern Math 1.959
DejaVu Math TeX Gyre 2.37
XITS 1.301

Regression

Last good Nightly: 2019-09-10
First bad Nightly: 2019-09-11
pushlog:
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=0e0e9c22321ed9f284744cfeb3507710b2d5d606&tochange=26711f10f5f45471b5856fb5cef08948f0e5bc21

Further bisection:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=0e0e9c22321ed9f284744cfeb3507710b2d5d606&tochange=9163b1b1da061853f0c34b2725937cbca6975def

Regressed by bug 1568227.

Here is another example from Wikipedia.
You need to install Native MathML to force MathML over images on Wikipedia.

If you resize the browser to give the math less space you can see that the parenthesis shrink.

What's happening here is that we're painting the old blob image to different sizes depending on the visible rect. There are a couple of options to fix this. It might be best to wait for the stuff in 1570081 to land first before attempting to fix this.

I'm currently leaning towards adding a way to resize a blob without updating its contents/dirty rect. Does that sounds like a reasonable approach Nical?

In addition, make sure the descriptor size stays in sync with the visible rect's size.
The descriptor's size stored in the resource cache is pretty much obsolete now, we should be able to clean it up and remove it.

The combined patches seem to be operating as intended. It looks like the remaining problem has to do with items moving around because there's not a reference frame keeping their coordinates in place like in SVG. Some thinking should let me figure out a solution.

This reverts some of the blob recoordination stuff that in retrospect didn't make sense and adjust things so that they work in the new world. In my very brief testing it seemed to make things work the way they're supposed to.

This seems to work: https://treeherder.mozilla.org/#/jobs?repo=try&revision=eb801f3709245ab5fff4202c48fb508d95c4d196

Now that we're painting based on the visible area we need to make sure that we
update the blob when ever the visible area changes. We'll do this by
unconditionally setting the visible area.

Backed out changeset 38e1e9c5d8c1 (bug 1580922) for causing webkit-text-stroke-property-001.html reftest failures

push that caused the backout: https://treeherder.mozilla.org/#/jobs?repo=autoland&selectedJob=269177577&resultStatus=testfailed%2Cbusted%2Cexception%2Cusercancel%2Crunnable&revision=38e1e9c5d8c1eba5abc6bc1f8129d0776fa68e32

backout: https://hg.mozilla.org/integration/autoland/rev/b91980649857354960d0ca5faf6a6a35aebd22b2

Backed out changeset 88a32122785b (Bug 1580922) for reftest failures at webkit-text-stroke-property-001.html.

Backout: https://hg.mozilla.org/integration/autoland/rev/ffd50b38a549f0cc41afdeb09edfcbb05965fabd

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=269285306&repo=autoland&lineNumber=19982

Looks like this was busted: https://treeherder.mozilla.org/#/jobs?repo=autoland&revision=c6552f7ec6a0febfbcfbea6800df6aac76d94968

Backed out 3 changesets (Bug 1580922) for wrench bustages.

Backout: https://hg.mozilla.org/integration/autoland/rev/249afaed4a5a73f5c349861eb0f3de03ded07e23

Push that started the failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&resultStatus=pending%2Crunning%2Csuccess%2Ctestfailed%2Cbusted%2Cexception&revision=c6552f7ec6a0febfbcfbea6800df6aac76d94968&selectedJob=269309944

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=269309944&repo=autoland&lineNumber=7579

Backed out 3 changesets (Bug 1580922) for wrench failures

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&fromchange=55e516f123f70d30db72b427af50c2aa33d45c6e&tochange=36847c53be73d10f49fe45891c37104291e389b9&selectedJob=269379498

Backout link: https://hg.mozilla.org/integration/autoland/rev/36847c53be73d10f49fe45891c37104291e389b9

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=269379498&repo=autoland&lineNumber=1122

[task 2019-10-02T07:02:44.274Z] Progress: 98% (975/986)
[task 2019-10-02T07:02:44.403Z] Progress: 98% (976/986)
[task 2019-10-02T07:02:44.413Z] Progress: 99% (977/986)
[task 2019-10-02T07:02:44.413Z] Progress: 99% (978/986)
[task 2019-10-02T07:02:44.429Z] Progress: 99% (979/986)
[task 2019-10-02T07:02:44.439Z] Progress: 99% (980/986)
[task 2019-10-02T07:02:44.439Z] Progress: 99% (981/986)
[task 2019-10-02T07:02:44.442Z] Progress: 99% (982/986)
[task 2019-10-02T07:02:44.442Z] ./wrench/src/rawtest.rs:628: Line is longer than 120 characters
[task 2019-10-02T07:02:44.442Z]
[task 2019-10-02T07:02:44.442Z] ./wrench/src/rawtest.rs:629: Line is longer than 120 characters
[task 2019-10-02T07:02:44.560Z]
[task 2019-10-02T07:02:44.634Z] Progress: 99% (983/986)
[task 2019-10-02T07:02:44.644Z] Progress: 99% (984/986)
[task 2019-10-02T07:02:44.666Z] Progress: 99% (985/986)
[task 2019-10-02T07:02:44.717Z] Progress: 100% (986/986)
[task 2019-10-02T07:02:44.717Z] Running the dependency licensing lint...
[task 2019-10-02T07:02:44.717Z]
[taskcluster 2019-10-02 07:02:45.054Z] === Task Finished ===
[taskcluster 2019-10-02 07:02:45.054Z] Unsuccessful task run with exit code: 1 completed in 105.672 seconds

https://hg.mozilla.org/mozilla-central/rev/0ea135d4a5c8
https://hg.mozilla.org/mozilla-central/rev/867a209cdbde
https://hg.mozilla.org/mozilla-central/rev/341f2bf17d9e
https://hg.mozilla.org/mozilla-central/rev/141b790d98a0

Hello,

I was able to reproduce this issue on Fx 71.0a1 (BuildID: 20190912215412) I can confirm that this issue is fixed on Fx 71.0b9.