I recently made the jump from Ubuntu (19.04) to Manjaro 18.04.2 and I am very happy with it!
The first thing that annoyed me though from this change was that Steam client wasn’t minimising to tray but instead it was both in tray and in taskbar!
The fix is simple!
Just open the terminal and give this command:
sudo sed -i '8s/=0/=1/' /usr/bin/steam
close steam and reopen it and from now own steam should close/minimise to tray and not to taskbar!
Alternative you can edit the file with nano by doing:
sudo nano /usr/bin/steam
and find in line 8
and change it to:
Press CTRL + X and Y then Enter and reopen steam!
Note: You might have to redo the above steps in case of a steam client update