When you drag the cursor to select a bunch of lines, as the cursor goes past the bottom of the SlickEdit window, the rate of line selection grows in a "runaway" manner and you lose control of the selection. The cursor then hits the end of the file (which may be quite large), and sometimes VSE hangs.
The rate of line selection should probably be slowed down, and lifting the mouse button should probably stop the rate of selection.
SlickEdit Pro 2023 (v28.0.2.0 64-bit Qt5)