I think it may make sense to rename "Change the selection" to "Toggle block selection mode". The concept of "block selection" is widely known outside of Mousepad (but not so for "change selection"). Corrections/thoughts?
Note that this feature is not available in the GTK3 build. https://git.xfce.org/apps/mousepad/commit/?id=48f37d339b9d2157cfca5a28aa62f3e93151901e
Closing this report.