Panic when a stacking context has an invalid transform.

[glennw]

If a transform matrix cannot be inverted, the webrender code will currently panic. Instead, it should detect this and not try to draw the stacking context.

[j3parker]

I can grab this. According to https://drafts.csswg.org/css-transforms/#transform-function-lists

If a transform function causes the current transformation matrix (CTM) of an object to be non-invertible, the object and its content do not get displayed.

Would a reasonable implementation in the context of servo mean that the object doesn't show up in the display lists (similar to if it had display: none)? Would that be done in layout?

[glennw]

It probably can be done in layout, however I think it's probably simpler to handle in WR. If we check in WR, it will also handle this case for any clients that aren't Servo.

This piece of code (https://github.com/servo/webrender/blob/master/webrender/src/tiling.rs#L1822) concatenates the full stacking context transform, and also marks stacking contexts as visible or not. I suspect all you need to do is check if the transform is invertible here, and if not, bail out of the function and mark the stacking context as not visible.

[j3parker]

Okay! I agree that putting the change in WR would be useful for other clients.

I originally asked because I had made the change you described and noticed that Servo still crashed because of hit-box stuff (i.e. that part of the DOM isn't rendered, but if you mouse in to the window then it panics.) I now think the correct solution is to do the check in multiple places. I'll PR the WR fix, open a related Servo issue this weekend.

[jdm]

This sounds like servo/servo#13266 and servo/servo#14806 too.

[notriddle]

It probably can be done in layout, however I think it's probably simpler to handle in WR. If we check in WR, it will also handle this case for any clients that aren't Servo.

Do non-Servo clients want that behavior?