For people complaining about black bars. Set static size to "no", continue with the rest of the guide.
May or may not cause other effects, such as an odd scaling making the "size" of the window behave odd in relation to your cursor and screen position.
This is hard to word. When using no static size, the window is the same size in the background, even though it's scaled to the res you set...
So say your window is 800*600, and you use the borderless tool to go to 1920*1440. It will zoom the 800*600 into that resolution. But the window itself is the same size.
Which makes your cursor feel extremely accelerated. Since it's move the same distance across the same window... But it APPEARS that it's moving further across your screen, since the window you visually see, is the larger one.
With no static size set, you can manually adjust the size of the window beforehand to the size you want. Which if you set a random anything to the res you want. Let's say a notepad window, using the borderless tool. You can drag your D2 window out to the same size (notepad is a reference size) then once at the correct size, press your hotkey on the borderless tool to set D2 to borderless.
This will now do the same thing I tried to explain above, but now the windows are the same size. Thus far removing that problem entirely.
It's an unusual fix, and a little annoying to do all the time. But it works well enough.