After selecting, the costume sometimes disappears

[Wilfredlam0418]

When editing a bitmap costume, when you release a selection, the entire costume (except for the selected part) sometimes disappears.

Steps to reproduce:

1. Create 2 squares. We'll call them square A and square B.
2. Drag square A.
3. Repeat a few more times and square B can suddenly disappear.

I am using a Mac to do this, so it may not happen when using other computers.