Reproduces on 2020.2.0a20.2611
Doesn't reproduce on 2020.1.1f1
Repro steps:
1. Create a terrain
2. Select the terrain and click the neighboring terrains tab, add a neighboring terrain
3. Go to the details tab and add a grass detail using the default particles texture
4. Paint from your terrain onto a neighboring terrain
Expected result: it paints smoothly and you can paint again
Actual result: on mouse up, both terrains are deselected, from this point clicking and dragging creates a rectangle selector instead of painting again. In order to paint again, you have to click the "activate this inspector button" AND reselect the detail to paint; you have to do this every time you paint a detail on one terrain to another.