Please,
make this a feature.
How is it possible that this is not possible?
I used to have the same suggestion but now what I think would be much simpler and consistent with the design of Shotcut’s multi-select is to just expand how multi-select is done with Shift so that you can select clips on multiple tracks while still holding down Shift. Doing that would then achieve the same effect as a box select in other video editors without having to draw a box.