Get a Bash that's not ancient. mapfile is there since version 4.0 from 2009.

Seems to be a old version of bash installed and used by default on macos

Even after updating still getting the same error

checked active bash version:

echo $BASH_VERSION

5.3.3(1)-release

What's relevant is whether "/usr/bin/env bash" runs the correct one.

I used "brew install bash && brew info bash" to get the path, then ran that shell (zsh doesn't work), then inside that new bash, ran the screensaver app.

I found the 4k fullscreen perf in iTerm2 to be not-great, so I did it again in the kitty (GPU powered) terminal macos app, and it was good.