Selecting objects using the border select tool on a big data set doesn't work correctly.
Tested on Windows 7 and 10.
I am sorry for the huge file size, but that is part of the problem I think.
Please load(will take some time)
In the left part of the screen, press B to border select as many of the cute triangles as you can.
Unfortunately, the selection shows up like this
So only the leftmost part is selected. It is even worse when there is more objects than in this file, once I only got a 2-4 object wide selection.