Entering a website an moving just the cursor suddenly moves the page up and down. No mouse buttons (left/right, scroll) involved. As 2.8.0 introduces gestures support this might be related.
Well, this problem seems to have solved itself after an update to gtk+-3.16.2. Previously installed was 3.14.10. I did not rebuild webkit-gtk after the gtk update, so this is a pure runtime issue. I won't close this issue because Webkit-gtk claims to be compatible with gtk-3.6.0 (3.12.0 with wayland), and this issue still should get fixed.