Clicking "Hide Sidebar" and "Show Sidebar" in the UTM.app UI sometimes hangs for a few seconds (shows the spinning beachball)

[rxhfcy]

Describe the issue
Clicking "Hide Sidebar" and "Show Sidebar" in the UTM.app UI sometimes hangs for a few seconds (shows the spinning beachball). Probably easier to notice if you click the buttons repeatedly.

Steps to reproduce:
0. Have lots of different kinds of VM's in the list on the left

1. Pick one of the VM's
2. Repeatedly click "Hide Sidebar", "Show Hidebar", "Hide Sidebar", "Show Hidebar" etc. etc. at least 10 times or so
3. Go back to 1 (pick another VM) and repeat until you see weirdness / stuttering / hangs

What happens: the animation noticeably slows down sometimes, for several seconds, shows the "macOS spinning beachball"
What I expected: no slowdowns
(also expected to see no UI animation because I have "Reduce Motion" enabled in macOS system settings, but that's another issue)

Seems to happen more on some VM's, less on others:

• macOS 13 Ventura aarch64 VM: animation does stutter, but less than the others I think
• Debian 12 (x64, QEMU): stutters a lot
• Arch Linux ARM (QEMU 7.2 ARM Virtual Machine): stutters a lot
• Fedora Linux aarch64 (uses Apple Virtualization): stutters a lot

Configuration

• UTM Version: Version 4.3.5 (87)
• macOS Version: 14.0 Beta (23A5328b)
• Mac Chip (Intel, M1, ...): M1 MacBook Air (16 GB RAM)

[rxhfcy]

Weird, now I can't reproduce this anymore. May or may not have something to do with playing with #5627 and deleting a weirdly named test VM.

The animations are definitely still not buttery smooth though (I'd be very surprised if they were 60 FPS)

[rxhfcy]

Pretty sure there's still something wrong here. Now I got beachballs when resizing the window (quickly resize it with the mouse to make it as small as possible and back, make it as wide as possible and back, as tall as possible and back, etc etc)