There's no need for a duplicate issue, this is already covered in #1025. Also as this repo is for bugs specific to the Linux port of the Steam client a general, platform-neutral option isn't really in scope here (although we keep a lot open for simplicity).
Nothing extracted yet.
There should be an option in the Settings -> Interface section to toggle closing the window vs quitting the application when hitting the
xin the top corner of the window.This should negate the need on linux to use the minimize-on-close workaround currently in place (see #1025) as users having issues will be able to disable closing to tray.
Another option may be to have a drop-down or radio option, where the user can choose the behaviour of the Close button; setting it to either "Close window, but keep open in the system tray", "Minimize window" or "Quit Steam"
Personally, I cannot see a reason to use the systray at all if the user chooses the minimize or quit options... Some other apps have a "Show icon in tray" option that can be set to things like "Always show icon", "Never show icon" and "Only show when closed to tray"