I check all settings and found an option for double click to maximize but it didn’t seem to work for maximizing windows by double clicking the title bar.
It’s the default settings I think, this is the behavior I have on my xfce that is almost vanilla 
In XFCE’s “Windows manager”, go in advanced tab, last knob is to define the action of the double click on a window bar, it’s set to maximize the window for me.
(Names could be wrong, I use XFCE in French and I’m not sure of the translations)
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It is curious… I have exactly this setting in Dom0, but it does not work for me.
There is a keyboard shortcut which functions, but not the double-click.
Yes. I went to the Qubes icon upper left corner → system settings → windows manager → advanced tab → scroll down to “double click action” and it has “maximize window” selected. But sadly it doesn’t work. quelle domage 
Yea it doesn’t work. I found somewhere in settings keybind for it though. I don’t remember where though.