ViewportCommand::Visible(false) doesn't appear to do anything

**Describe the bug**
Ref: https://github.com/emilk/egui/discussions/3885, couldn't find a similar issue

I'm porting my app from `0.23` and ever since viewports were introduced I've been unable to hide the app's window while keeping the thread running.

I have tried adding `ctx.send_viewport_cmd(ViewportCommand::Visible(false));` to the top level of the `update` method itself to see if anything would happen, but neither that nor triggering it sporadically/via some other method seems to do anything aside from printing the "processing" log.

I'm using NixOS which *may* have something to do with it, but the same app packaged the exact same way works when on `0.23` just fine via `frame.set_visible()`.

The code used for testing in both versions is
- `23`: https://github.com/4JX/L5P-Keyboard-RGB/tree/main/app
- `26`: https://github.com/4JX/L5P-Keyboard-RGB/tree/egui-update/app
- `26-minimal`: https://github.com/4JX/L5P-Keyboard-RGB/tree/egui-update-minimal/app

**Expected behavior**
The app window is hidden while the program keeps running, as it did previously.

**Screenshots**

**Desktop (please complete the following information):**
- OS: Nixos 24.05, 6.5.13, x86_64
- Version 23/26

Start with the linked 26-minimal app and the update method where ctx.send_viewport_cmd(ViewportCommand::Visible(false)) is called. Compare its behavior with frame.set_visible() in the linked 0.23 app, focusing on the NixOS environment. Done means the viewport becomes hidden while the application thread continues running.

Written by the indexing model from the issue text.