Bug 1528180 fixed the non-root-scroller testcase. I wrote this test-case to debug something else... And in WR it jitters quite a lot.

Debian Testing, KDE, X11, Macbook Pro
Same range as bug 1541072 comment 11. Jittering was introduced together with scroll anchoring because behavior was different before.

So this fixes it, but it's unclear whether it's the right fix:

diff --git a/gfx/wr/webrender/src/spatial_node.rs b/gfx/wr/webrender/src/spatial_node.rs
index 64088f3faa17..3ff9f9675f95 100644
--- a/gfx/wr/webrender/src/spatial_node.rs
+++ b/gfx/wr/webrender/src/spatial_node.rs
@@ -623,7 +623,10 @@ impl SpatialNode {
 
     pub fn scroll_offset(&self) -> LayoutVector2D {
         match self.node_type {
-            SpatialNodeType::ScrollFrame(ref scrolling) => scrolling.offset,
+            SpatialNodeType::ScrollFrame(ref scrolling) => LayoutVector2D::new(
+                scrolling.offset.x.round(),
+                scrolling.offset.y.round(),
+            ),
             _ => LayoutVector2D::zero(),
         }
     }

Glenn, we seem to round scroll offsets in some places in some places but not others, does the above make sense? Should I instead find the origin of the unrounded number?

I think it's not the right fix, although I'm not 100% on that. Could you try disabling gfx.webrender.picture-caching on your machine, then re-test (after a restart) and report if that affects whether the bug occurs?

No jittering with picture caching disabled.
mozregression --launch 2019-07-15 --pref gfx.webrender.all:true gfx.webrender.picture-caching:false -a https://bugzilla.mozilla.org/attachment.cgi?id=9078458

Any chance you could post a video of this? I'm having trouble seeing it on my machine - it might be quite reliant on dpi / resolution etc to make it occur?

Win10, GTX1060, 2560x1440, Dell UltraSharp U2515H (117.5 DPI)
(Also reproducible with: Debian Testing, KDE, X11, Macbook Pro A1502)
left: mozregression --launch 2019-07-15 --pref gfx.webrender.all:true gfx.webrender.picture-caching:true -a https://bugzilla.mozilla.org/attachment.cgi?id=9078458
right: mozregression --launch 2019-07-15 --pref gfx.webrender.all:true gfx.webrender.picture-caching:false -a https://bugzilla.mozilla.org/attachment.cgi?id=9078458

Even text glitch (bug 1565473) and diagonally torn content (bug 1541072 comment 19) are visible at 6s when you suddenly see white lines blinking once.

This may be fixed by https://bugzilla.mozilla.org/show_bug.cgi?id=1566712.

Jittering looks fixed. The other issue I found is bug 1566178 comment 7.

Should this one be marked fixed, it seems the other issue above is something different?