https://bugs.winehq.org/show_bug.cgi?id=38409
--- Comment #47 from Alexey Min alexey.min@gmail.com --- @Robert Walker I just tested windows steam client with unpatched wine 3.2 (from git) with Ubuntu 16.04 (Plasma 5.8.8, KF 5.36, Qt 5.6.1) and it works fine. With xembedsniproxy running.
Steam Client can ONLY minimise to a system tray (not to a Desktop taskbar / panel)
Not only Steam window can be minimized to taskbar and restored from it, but it can also be successfully restored from tray icon, when steam window was actually closed (disappeared from task bar panel). So, this whole patch is not so important, I guess: https://i.imgur.com/8CbP236.png (blurry icon is a consequence of using xembedsniproxy).
I suggest you to try vanilla wine from git (no patches) and Plasma with legacy-systray (xembedsniproxy). If it does not work, you can killall xembedsniproxy, restart wine and a separate wine systray popup window should appear, it should always work as last resort.
I can try to test it with Gentoo and Plasma 5.12 some time later today, tomorrow or at least this week.