Dear Maintainer,

I filed an upstream bug report and closed it, because upstream has solved the issue
https://gitlab.gnome.org/GNOME/simple-scan/-/issues/550

   * What led up to the situation?

     Upgrade from Debian-oldstable to Debian-Trixie updated simple-scan from v45.5 to v46.0

   * What exactly did you do (or not do) that was effective (or
     ineffective)?

     compiled upstream version v46, confirmed it had the bug
     compiled upstream version v49, confirmed the bug got fixed

   * What was the outcome of this action?

     bug fix is known

   * What outcome did you expect instead?

I have the same issue. I would like to add that the horizontal scrollbar reappears when you force a window re-render, for example by minimizing
and restoring/maximizing the window. After that, the horizontal scrollbar for the scanned pages becomes visible again.