You should definitely say "monitor" not "screen" as the latter is easily confused with "Screen", which as Bryce points out, often has several monitors.  The only case where there are multiple root windows is where there are multiple Screens, each with its own root.  Multiple Screens is generally a less flexible configuration than Xinerama, except that it does allow monitors (and usually multiple video cards) with different colour depths and Visuals.  A multi-Screen configuration is basically a set of independent displays with a common keyboard and mouse, but supported by a single X server.  Rarely seen these days.

On a broader issue, I think you are trying to tackle too narrow a problem.  The resolution changing problem is a special case of a more general situation - something wants to take the display hardware (or some subset of it) away from the Window Manager, perhaps also the X server, and existing clients, without altering their stored state.  This is exactly what happens when you switch VTs, which is already well supported.  (In fact switching VTs seems a reasonable work-round for the full-screen-with-resolution-change problem, at least when VTs are available.)  The only difference I would see is that you want applications to take a subset of the monitors in some cases.  Looked at like this, asking the WM to change the resolution looks too specialised.  It should be OK for the application to change the resolution, so long as the WM knows to do nothing about it.  For this to work well, the solution involves both the WM and X server, but that was probably true of your original proposal.

> ...  figuring out how to specify a monitor to go fullscreen ...



-----Original Message-----
From: wm-spec-list-bounces gnome org [mailto:wm-spec-list-bounces gnome org] On Behalf Of Bryce Harrington
Sent: 01 November 2012 11:57 AM
To: Ryan C. Gordon
Cc: wm-spec-list gnome org

On Thu, Nov 01, 2012 at 12:26:00AM -0400, Ryan C. Gordon wrote:
> Two other unrelated n00b questions from me, to make sure I'm understanding the system correctly:
> - Should all the places we say "screen" in this spec really say "monitor"? I'm using a lot of overloaded X11 vocabulary interchangeably, and I probably shouldn't.

On a multi-head RANDR system, there is a single "Screen" stretched
across the multiple displays or monitors.

> - Is there always one root window per monitor? I have text for setting a root window property, and it's more useful if it can note the current fullscreen window for that monitor (to cover Martin's concerns). If there's only one root window that covers all monitors, I need to change this to be an array. My understanding is that Xinerama stretches the root window across multiple monitors, but my reading of XRandR is that this isn't true there. You can see the text in question, in both forms, here:
> ... the _NET_RESOLUTIONS property as it currently stands is only useful if the app can effectively enumerate multiple root windows, too, or if the app doesn't care what monitor it lands on.

RANDR uses one screen for multiple monitors, and one root window for
that screen.  Thus one root window for all monitors.

Xinerama uses multiple screens which are stitched together.

I don't know about use cases where there are multiple root windows in an
X session, but that would be an uncommon layout at best.

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