Tested on Nightly 55.0a1(2017-03-28) with Mac OS X 10.12, Ubuntu 14.04
1. Go to http://conorlastowka.com/book/CitationNeededBook-Sample.pdf
2. Open print preview
3. Drag the scrollbar up and down
The rendering is smooth without any issues.
Some flickers appear when you move up and down the scrollbar.
If I turn off from about:config the apz.drag pref. everything works as expected, this behavior can be obtained if you turn off the apz.allow_checkerboarding.
How do you open the print preview on OS X? As far as I know we never show the Firefox print preview window on OS X, since we use the OS-provided preview tool instead.
Sorry, I wanted to say that is reproducible only on Ubuntu. You are right there is no print preview on OS X.
The flickering appears to be checkerboarding, so this is probably a dupe of bug 1251617. However I also notice that when scrolling the print preview window in Linux, the content seems to shift where it is on the preview "pages" depending on the scroll position. That's really weird. Seems to happen with and without apz.drag.enabled, and happens with wheel scrolling as well. That's probably worth a separate bug.
I filed https://bugzilla.mozilla.org/show_bug.cgi?id=1352083 for the other issue.
I'm going to dupe this to bug 1251617, which is what we're using to track checkerboarding during scrollbar drag.
*** This bug has been marked as a duplicate of bug 1251617 ***