ENA-148: Highlighted blocks are blurry on non-retina display in some situations

Expected Behavior

Blocks should always render crisply.

Actual Behavior

In some situations, the blocks become blurry. I had seen some reports of this on the forums, and stumbled on an example while viewing a kid's project in Chrome on my non-retina display. I noticed that the blurriness happened on a "when left arrow key pressed" stack that was highlighted because it was running.

Steps to Reproduce

1. Use a non-retina display
2. Start from the default project, and add this stack to the cat sprite:
   
3. Press the left arrow key to run the stack
4. Note that the highlighted stack is blurry:
   

Notes

• The blurriness does NOT occur if you use "when space key pressed"!
• It DOES occur for left, right, up or down arrow
• It DOES NOT require pressing the key itself - you can click the stack to run it (if it starts with an arrow key pressed hat), and get the blurriness
• It does NOT occur for any other key
• It does NOT occur when using the green flag or sprite clicked hats
• It DOES occur for certain other hats, including "when backdrop switches to backdrop1"

System Details

Chrome 95, Mac OS 10.15.7

Reproduce the issue in Chrome on a non-retina display using the default project, the arrow-key stack, and the listed hat variations. Compare highlighted blocks with non-highlighted blocks across the reproduction cases; done means the affected highlighted stacks render crisply without changing the cases that already render correctly.