some people don't know that, and it isn't made obvious, so having it by default on is great i think... but, enabling it at random for no reason? that's ridiculous
you had enabled "render region", which means blender will only render a small selected area, which is probably what that yellow square you're seeing is (since the area is very small, and blender highlights it in yellow)
to disable that, you can go into view in the sidebar and uncheck "render region", or just hit CTRL+ALT+B
if i get some spare time i'll throw some rounded corners on some of my recent web designs that i'm allowed to show, though i've thought about it more and i don't think that's my main issue with it. i feel it makes it feel like websites are more so just that, little pages in an app, when they can be, and often times are, so much more. i like when they can take their whole screen of space, without any borders, cut edges, anything like that, which is why i personally use a theme which even hides the tab bar behind a hover. i like to treat websites as apps in their own right, and putting them into a little box just doesn't sit right with me. if it didnt have those borders, and were just rounded based on the normal windows border radius, i'd likely be fine, but i feel this puts too much connection between the browser and the site
i've designed a few websites recently which really favour sharp corners, and when one of my sharp objects randomly has a rounded corner, when none of the others do, just because it happens to be in the top left corner, in my opinion that's a bad thing?
also made it way more accessible to newcomers, i tried to use it with 2.79 but i just couldn't get a hang on the UI, 2.80 really made it usable for me and many others
beyond one or two mistakes of my own doing, where i didn't read or think before running a command, linux is perfectly stable compared to windows for me atleast!