Only the bubbles, though. The text input field and the sidebar are standard Cocoa controls (at least that was the case when I last checked).

It's not as bad as a full web app

I get a beach ball All-the-time with Messages lasting 5-10 seconds. I struggle to imagine that anything it’s doing is that computationally intensive.

I suspect this might be a problem with your installation - if you create a new MacOS user account for testing, does the problem persist? I don't recall Messages every beach-balling for me and I'm not exactly on new hardware.

Tangentially, this appears to be the general (and usually, only) troubleshooting step you can do with Apple products.

Yes, creating a new user resolves the problem. Yes, resetting my device to factory settings without restoring from backup resolves the problem.

How do I resolve the problem without deleting my entire history?

So if you have problem that goes away with another user, the problem commonly lies in a corrupted preference related to the app, so you start hunting out the .plists for the application and moving them to the desktop before relaunching and seeing if the problem is fixed.

If not, then you start poking about in Application Support for the app.

After 9 years of Apple use, I have successfully made this method work one time (iMessages), and even then, it took a full system reboot after each file move to validate whether or not that had made a difference.

The whole process took the better part of a day. It was long, it was tedious, but at least I got there in the end. There was no paid-support story that would have done this for me, but hey ho, it all worked out. I can't imagine I'll ever have the time to go through that much effort again.

The same cant be said for my iMessage history on my iPhone, which has been lost repeatedly as the only solution was "don't restore from your backup".

Ah - I used to use it fairly frequently in the good old OS X 10.2-10.6 days - for various apps. Seemed to work fairly well.

Really? I thought it was based on old iChat application, since it used to support other IM providers.

Support for that was ripped out years ago.

I know support for those services is long gone, but I thought it was still based on the same code.

The last time I saw support for it still there, the IM stuff would open in a separate, old-looking window from the Messages window. Basically, they kept the old code and just built a new version parallel to it, then eventually took out the old one.

It’s still a web view in Catalina as well.