A DESCRIPTION OF THE PROBLEM :
The Aqua L&F includes some unnecessary repaint logic (and listeners) that we can safely remove.

The root pane's default button used to continually pulse/repaint in Aqua.

According to https://en.wikipedia.org/wiki/Aqua_(user_interface)#OS_X_Yosemite,_El_Capitan,_macOS_Sierra,_High_Sierra,_Mojave,_and_Catalina :
> A blue button is the default action, and in OS releases prior to Yosemite, would appear to pulse to prompt the user to carry out that action.

But Mac OS Yosemite was released around 2014. Apple has long since discontinued support for OS's that had this feature.

As a related bug:
At one point I filed https://bugs.openjdk.org/browse/JDK-8344697 to point out that AquaButtonUI's AncestorListener could cause a noticeable performance slowdown. But other (seemingly unrelated) improvements made the performance acceptable again.