One of the features of LBB is that the WHEN mouse events work in a window of type WINDOW as well as a window of type GRAPHICS. This allows you to do some things that would not be as straightforward in LB 4.5.
Here's a very simple demo of a window which is split into two texteditor controls (they could just as well be any other kind of control). You can move the split point by dragging it horizontally with the mouse. It uses an embedded BBC BASIC statement to switch the cursor to a double-pointed-arrow, something which isn't so easy in LB (would anybody like me to add some more cursor types to LBB?).
WindowWidth = 600
texteditor #w.te1, 0, 0, 290, 300
texteditor #w.te2, 294, 0, 290, 300
open "Pane test" for window as #w
#w "when leftButtonMove resize"
!MOUSE ON 132
sub resize h$, x, y
#w.te1 "!locate 0 0 ";x-2;" 300"
#w.te2 "!locate ";x+2;" 0 ";WindowWidth-x-18;" 300"