Mouse cursor should be in the window as shown in this screen shot.
Do you have the latest version of DEVONthink (3.9.6 I think) and macOS (varies by Sonoma, Ventura, etc.). Some features of Sonoma has been buggy so far, so perhaps that the issue.
Also in macOS System settings check how your Mouse and TrackPad are set to react to gestures.
As it works for me and others, it’s probably not DEVONthink, maybe macOS and maybe your devices. A horizontal scroll bar will just take up more valuable screen space, IMHO.
To work there must be more columns displayed but not visible. For me with iMac with Apple wireless mouse, the gesture is 1-finger left/right swipe. For my iMac and MacBook with Apple Trackpad the gesture is 2-finger left/right. Why different? Dunno, and not going to explore further.
