Integer underflow in clamp_scroll

[sternenseemann]

Describe the bug

When moving beyond the screen space with the arrow keys in the debug build, it crashes due to an integer underflow. The release build behaves as expected (presumably because the check is not emitted).

To Reproduce

> base64 -w 0 /dev/zero | head -c 200 > test
> ./target/release/ad test
# Move to the end of the line, no problem
> env RUST_BACKTRACE=1 ./target/debug/ad test
Fatal error:
panicked at src/ui/layout.rs:796:28:
attempt to subtract with overflow

Versions & OS Details

• Linux NixOS/unstable
• ad Version f2e781c

[sminez]

With the recent changes merged in #58 this no longer panics but the highlighting behaviour for selections isn't correct so that needs addressing.