Window.Resizing
From Xojo Documentation
Event
The user is in the process of resizing the window.
Notes
Resizing executes regardless of whether the LiveResize property is True. Note: The Top, Left, Width and Height properties have already been adjusted to account for sizing at this point if LiveResize is on, but the controls haven't drawn themselves. You could, for example, use a Boolean property to indicate whether a Canvas control should redraw in its Paint event.