Bottommost line cleared and all preview frames toggled on.

[HustleDragon]

When doing almost anything after spending about 30 minutes in a session, the very bottom line (with pixels) of each frame erases completely, and all frames automatically toggle on preview.

Steps to reproduce the bug

1. Go to piskelapp.com
2. Create any sprite featuring 2 frames or more
3. Add any pixels to the bottom line
4. Toggle off preview of one or two frames
5. Wait about 30 minutes and try doing several (or any) different actions

Environment details

• Operating System: Windows 10
• Browser: Google Chrome v75.0.3770.142

Sprite details

• Number of Frames: 29
• Sprite Resolution: 64x65px
• Session Duration: Approx. 1h 30min

Here's the .piskel sprite I was working on, compressed to be uploadable:
Princess Stella WIP Spritesheet-20190722-235431.zip

[PoolloverNathan]

common theme: only happens if a frame is hidden