Comment 11 for bug 1817965

It is not specific to icon buttons. It can be reproduced in "normal" grid cells as well.

There's some sort of timing issue going on. After the first drag the mouse is a pointer over the cell when it should be an I-beam. If you wait for it to switch to an I-beam (by moving it in and out of the cell), the second drag works fine. If you don't, the second drag produces the assert.