There are two user-visible changes in here:
1. If a scroll region is set such that there is a bottom margin and no
top margin, scrolling the region forward used to discard the top
lines. Now those lines are appended to the scrollback.
```shell
# Assuming a terminal window with 24 lines.
printf '\033[H\033[J\033[3J\033[0;5r' && seq 100
```
This command used to result in an empty scrollback. Now it contains
the numbers 1-96.
2. If a scroll region is set such that there is a top margin and no bottom
margin, scrolling the region forward used to append the top lines to
the scrollback. Now these lines are discarded.
```shell
# Assuming a terminal window with 24 lines.
printf '\033[H\033[J\033[3J\033[2;24r' && seq 100
```
This command used to populate scrollback with the numbers 2-78. Now
the scrollback is empty. The numbers on the screen are the same as
before: 1 and 79-100.
Related issue: #3113.
620eaf1ada