If I understand what you mean, there’s a very thin horizontal line on a track. Dragging a clip to the track and releasing the mouse above the line pushes all other tracks out of the way of the new clip. If you want the track to overlay on top of others underneath, release the mouse below the horizontal line.
Another idea is to move the head in the right position for the clip on the timeline, then drag the clip over to the canvas window (which will make a few options visible). Drag the clip to one of the options.