On Mac, I've noticed that, when Servo isn't the window 'in front', then even though you can still scroll Servo because it's still viewable, it is always the same 'layer' (in the sense of the Webrender layer) that scrolls, even if you move the cursor onto a different part of the page.

So for example taking https://output.jsbin.com/lukiye, if you start by scrolling the main page, then move Servo to the back (for example take the terminal in front of it), while still showing Servo. You can still scroll the main page, however when you move your cursor onto the div in the middle of it, trying to scroll it will just scroll the page, not the child div. Same thing if you start by first scrolling the child div.

In other words, the scrolling is 'stuck' in the layer that you happened to be scrolling the last time that Servo was still 'in front'.

I've checked, and when Servo is no longer 'in front', the scroll in webrender always receives the same 'cursor', even when you actually move your cursor around...

I've also noticed that when Servo isn't in front on start-up, you cannot scroll at all.