Well this is 100% reproducible on all 20+ machines I use, both VM and real PC's. fluff is the only app effected. Ctrl+Alt+M does not share this effect. Occurs when dragging the right side window edge, lower right corner or window divider bar in a rightwards direction.
Note the effect occurs less in real PC's and is more often reproduced when moving the divider bar to the right.
Anyhow this is only a minor issue.
and from another pc
other really awkward effects also only associated with fluff is the use of a Bluetooth mouse + keyboard on a live machine in which the mouse only controls are reversed, ie: selecting and holding a mouse button down brings up the desktop menu, then one has to scroll to the desired menu item then release the button for selection. holding down the mouse button while finding the menu item is required or the menu will vanish. lol really confusing to use i can assure you.
am using flwm_topside almost exclusively (albeit a copy of Fluxbox around here somewhere).
Note: Updated with corrected info.