Problem: Test for 'smoothscroll' is ineffective. Solution: Change the order of testing "zb" and "zt". (Luuk van Baal, closes #12366)
Problem: Cursor positioning and display problems with 'smoothscroll' and using "zt", "zb" or "zz". Solution: Adjust computations and conditions. (Yee Cheng Chin, closes #11764)