Stack Glow causes text in blocks to wiggle on Firefox

[BryceLTaylor]

Expected Behavior

When a stack is glowing the text on the block stays in the same place as when it's not glowing.

Actual Behavior

It moves to a slightly different position. When the glow flashes on and off rapidly this makes the text wiggle.

Steps to Reproduce

Load this project: https://llk.github.io/scratch-gui/develop/#216001494
Run it
Note: the "When I receive [start]" block texts moves slightly.

Operating System and Browser

Firefox on Mac and Windows

[towerofnix]

Can repro this on Linux too. I've never opened an issue but I've definitely noticed it for a very long time, so it's not a recent regression.