Tested in v7.83 & v7.83 betas,
On Windows 7 with a 1920x1080 screen size.

When a custom window has a listbox the horizontal scroller will not resize correctly for very long lines.
This affects custom windows where the whole window is a listbox and windows where only a portion is a listbox.

Code
alias show_scroll_error {
  window -al @scroll
  aline @scroll $regsubex($str(a,800),/./g,\n $chr(32))
  window -b @scroll
  echo -s l: $line(@scroll,1)
}

Scroll to the right and you will see (on my system) the number 431 instead of 800
The echo confirms that the line does extend to 800.