Steps to reproduce:

Method A
1. Open attached project with latest trunk rev 5181ec54d595 plus daa5ab5ce3d7 grafted on top to make HDRP work.
2. Observe that the lightmap is updated progressively even though the checkbox for progressive updates is unchecked.

Method B
1. Open any existing project
2. Make sure that the lightmap resolution is set to a relatively high value (128 and up)
3. Make sure that Progressive Updates toggle is off
4. Generate lighting using either the CPU or the GPU lightmapper
5. Notice how the viewport is progressively updating

Notes:

- Tested on Windows10 using a Geforce2080Ti and TITAN V.
- Reproduces in: 2020.1.9f1, 2020.2.0b6, 2021.1.0a1, 2021.1.0a2
- Reproduced both with the CPU and the GPU lightmapper
- Reproduces when lightmap resolution is high; sample counts do not seem to have any effect