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If I have understood the tutoriel, there is a workspace on each physical screen.
Yes.
I don't understand why I can't change the focus with mod-1, mod-2 whereas I can change with mod-e , mod-w ...
That's how it used to work. If you were on one screen and workspace 2 was on the other one, then mod-2 would switch focus to the other screen. That changed a week or two ago, and now you stay on the same screen and the workspaces get moved as necessary. So mod-2 still changes focus to workspace 2, but it'll move the workspace rather than moving focus to another screen. I found the new behaviour rather disturbing (especially since it had a bug if you have defaultGaps different for the screens, but that's now fixed), but I'm getting used to it. I think the current behaviour's probably better. The switching around of the workspaces is still a little disturbing, but it fits well with the general feeling of this kind of window manager (where windows get moved around as appropriate), and it makes mod-{1,2,...} much more predictable. It's quite possible the tutorial hasn't caught up.