React 19 support

[valerauko]

I couldn't find an issue explicitly tracking this.

The README only states that reagent doesn't support 19, but it's unclear if this is a temporary thing or there is no plan to support 19 at all.

I don't know what changes are necessary for react 19 support.

[Deraen]

Biggest thing (or the first thing at least) has been to be able to run the test suite successfully with React 19 which was problematic because it turns on Concurrent Mode: #626

Test suite is now OK (with a few workarounds for some cases), I'm now considering what to do with cljsjs tests (I'll likely just drop testing with them as React 19 isn't available as a UMD files anymore) and if I need to replace those tests with something else. Then it will need more testing on real projects to figure out if something still breaks (and to document some known cases that cause problems.)

React 19 support is now planned and I have time to work on it, but I haven't had time to make posts about it yet: https://www.metosin.fi/blog/2025-04-24-reagent-and-react-19-support

[Deraen]

Version 2.0.0-alpha1 is now available: https://github.com/reagent-project/reagent/blob/master/CHANGELOG.md#200-alpha1-2025-06-24