In Windows (as in I think most OS’s) you can resize a window by clicking and dragging any of the four sides.
I think that in Windows 10 they have pretty much removed the window border and instead have a very thin line. However my understanding is that there is still an invisible 10 pixel area that you can grab.
However when running Brave in Windows the top edge grab area is tiny and very hard to click on. It seems to be literally one pixel.
Note that I still get the thin border when running on Windows 7, so I’m not sure it is a windows version specific thing.