This actually designates an area where you can click and drag to resize the window. I'd argue that this behavior has become a ubiquitous expectation because of this design.
Like, a 1px border isn't too back when I have a keyboard shortcut that makes a full 1/9th or so of the window's area the effective border. But when it's exactly a pixel — and combined with macOS's cursor's hotspot not being on the cursor tip … it just takes what should be simple to way harder than it need be.
And then there are the too skinny to grab scroll thumbs, which are also invisible without gratuitous scrolling. And they disappear quite quickly.
Three alternate solutions:
1) Long hover (1/4 sec?) near window edge, to make edge grabbier. Long hover (1/4 sec?) near scrolling side for scroll bar to appear.
2) Pressing one of the modifier keys to make edges and scroll bars appear and be grabbable.
3) Toggle keyboard command, that makes edges slightly thicker and holds scroll thumbs visible, vs. hiding them.