Related issue: #31393

So far, the workgroup for the compute shader consists of a scalar and the dispatchSize always uses dispatch.x up to device.limits.maxComputeWorkgroupsPerDimension before dispatch.y is used. The default workgroup is now [ 64, 1, 1 ] instead of [ 64 ]. This ensures that all previous examples are treated as before.

Example on webgpu_compute_particles_rain, usual usage:

computeParticles = computeUpdate().compute( maxParticleCount );
renderer.compute( computeParticles );

Use with workgroup and dispatchSize

computeParticles = computeUpdate().computeKernel( [ 16, 16, 1 ] );
renderer.compute( computeParticles, [ 10, 10, 1 ] );

Since the instanceCount in the example is 25,000, workgroup = [ 16, 16, 1 ] and dispatchSize = [ 10, 10, 1 ] = 16 * 10 * 16 * 10 * 1 = 25,600 threads, enough to cover everything. If you choose smaller values for dispatchSize, e.g., [ 9, 9, 1 ], you would see that not all raindrops are computed.

You now have full control over the thread distribution for the GPU, which was not possible.
I'm setting the PR to draft for now, even though it works, because I might think of a few more things I'd like to add. Maybe warnings if you enter something incorrectly.