Main viewport blocked from updates while deferred viewports are rendering.

[hydra]

Describe the bug
When the mouse pointer is being moved over a deferred viewport the main viewport does not update.

To Reproduce
Steps to reproduce the behavior:

1. run the updated example in PR Update the multiple_viewports example to show how to use Context::request_repaint_of(). #7676 (source: https://github.com/hydra/egui/blob/improve-deferred-viewport-example-1/examples/multiple_viewports/src/main.rs)
2. observe that when the mouse pointer is being moved over the deferred viewport, the main viewport does not update, even though there's a background thread requesting a refresh of it via the context. request_repaint or request_repaint_of(ViewportId::ROOT).
3. observe that when the mouse pointer is being moved over the deferred viewport, the main viewport does not update, even though the main thread is using request_repaint_after(duration)

Expected behavior
Main viewport is not blocked from updating.

Video

https://youtu.be/hj1myK0yVDQ

Desktop (please complete the following information):

• OS: Windows 11 24H2