Strong +1 on doing this as an Interface setting rather than redefining the close button outright. An "Action on close" selector defaulting to current behavior, lets everyone do what they want.
Two details worth adding for anyone landing here:
The old STEAM_FRAME_FORCE_CLOSE=1 workaround no longer works on the current client. I set it and confirmed it was actually present in the running process's environment, but clicking X still just minimizes. So right now there is no reliable way to get exit-on-close at all.
It isn't only the X button. Alt+F4 doesn't quit Steam either, it also just minimizes to the tray. Alt+F4 is the standard "close this application" shortcut on basically every OS, so having it silently minimize instead is its own small annoyance.
Another Strong +1 for me too. It's ridiculous that this issue hasn't been addressed since 2013. I'm a paying customer who regularly buys new games. Do you guys really not care about keeping customers satisfied by implementing even the most basic features?
Nothing extracted yet.
I have been reading Bug #1025. But the idea here, instead of changing the behavior of the close button, why not add an option in the interface tab of the settings?
By default, It can be something like the following and the user can choose which he wants:
Action on Close:
Probably the wording would be changed a bit, and the user can only choose one option, but the general idea is for it to be easier for users to change it than having to change an environment variable.