A new version of river has been released, read the ofiicial release notes below:

This bugfix release fixes a regression introduced in 0.3.6 that prevents adaptive sync/VRR from working properly. It also fixes an assertion failure that can be hit with ~50 days of uptime.

Full changelog:

Isaac Freund (4):
      build: bump version to 0.3.7-dev
      Output: check scene damage before rendering
      river: wrap monotonic time > 2^32-1 milliseconds
      build: bump version to 0.3.7