After a bit of digging i found this issue was indeed caused by glamor in x server 1.18.2 and it has a fixed bug in xorg:
https://bugs.freedesktop.org/show_bug.cgi?id=94554 Debian should apply this patch since it affects quite some web pages.
- Bug#818179: This issue is resolved upstream in xorg Laszlo KERTESZ
- Bug#818179: This issue is resolved upstream in xorg Martin Dickopp
