As for how to fix the screen being cut away, when you are making a build there is option(In player settings) to limit displayed resolutions to just some aspects.(For sandalone) Not limiting aspects was causing similar effect im my games,As for openGL you jsut up same resolution you want it to have in Embeded window.I solved this problem by using lower resolution than my actual resolution - jsut same aspect.
For example im using 1360:768 which is same aspect as 1920:1080 that i work on.
If embeded window has smaller size than your chosen resolution it cuts off.
but i think there is something more than that, your canvas is set up wrong.