If anyone wants rounded corners in telegramDesktop, you can download the just released version from my github: https://github.com/Sventra/tdesktop

The only change to it is that corners pretty much everywhere are more rounded, from the large/small 6/3 to 10/8
I'm still unaware of a way to have this as a selectable slider, if anyone wants to help, just DM me or join the beta testing group