Double vertical scrollbars in DevTools panel when OS is set to show scrollbars

[matatk]

I think this could be due to the use of flexbox, which isn't needed in the DevTools panel.

[matatk]

I tried to fix this in #418 but in Firefox the extra scrollbar was always there, as if the panel itself was taking up slightly more than the available vertical space. Setting it to both 100% and 100vh (both html and body) didn't help, though, and the problem only occurs on Firefox (out of Firefox and Chrome).