screen number and modal dialogs

[high_flyer]

According to the docs "correctly" should be "1", not "2", so if there is a bug, it's in numScreens().

Why is that?

int QDesktopWidget::numScreens () const

Returns the number of available screens.

See also primaryScreen().

And I have 2...

Besides, Qt should have nothing to do with choosing the screen to open the window on (at least not "by default").

Well not with choosing the screens, but delivering the right information about the location of the widget - and allowing to choose the screen on which it should live.
And (specially) under KDE I would expect this to work - since KDE is built on Qt.
But I will read some more and look around.

[high_flyer]

Ok, for the benefit of others who might run in to issues of that sort:
The reason I thought that its either Xinerama or clone mode is because under SUSE (which I am using) SaX2 gives only these two options.
So if you are under SUSE (currently I am on 10.1) you will have to edit your xorg.conf manually, so that the you have a "Device","Screen" and "Monitor" scetion for each screen you have.
At least with NVidia, which I use, there is an option "TwinView" which you have to comment out - and Xinerama ofcourse as well.

So now I have an X server running for each screen.
Obviously the above problem is gone but I still have to see how I will do this on the QuadView machine I am coding this for.
Its a Matrox Quad View card - not NVidia, and obviously it has no "TwinView" option.
But that is something that will have to wait for now.
At least I know what I will be looking for

Hope this is of use to someone.

[wysota]

So it was Xinerama causing it or a yet unknown reason?

[high_flyer]

Xinerama makes the whole desktop as one screen.
I think thats the reason why the screen number always returned 0, on both screens.
But the way I understand the docs, the screen number should return correctly even in xiberana mode.
Either I got it wrong, or Qt has a problem with this issue.

[wysota]

I'd say it's a good candidate for a bug report. Either way something is incorrect here.

[high_flyer]

Oh by the way - xinerama confuses gimp as well.
If I start gimp on screen 1 - all the other dialogs it wil open on screen 0.

[wysota]

So it looks like the natural behaviour and the problem seems to be that Qt returns an incorrect number of screens or sth like that.

[high_flyer]

Well, yes and no.
As far as I understand the docs, Qt should return the number of physical screens, regardles if this is a viratual desktop (xinerama) or two desktops, as you can see in the illustration in the DesktopWidget docs.
Besides, the docs text it self makes no conditions on when numScreens() should return one screen (for xinerama) and N screens when not using xinerama (or virtual desktop).

All windows opened in the context of the application should be constrained to the boundaries of the primary screen; for example, it would be inconvenient if a dialog box popped up on a different screen, or split over two screens.

The text in the quote is talking from the exact problem I have (popping dialog box on another screen than parent).
So Qt is supposed to allow control over this situation, and allow the user to target physical screens on a virtual desktop - but it doesn't - not in the case I posed in this thread at least.

[hvengel]

Having done some work that involved knowing what physical screen my dialog was on I have learned something about how this works. There are also some misconceptions in this thread that I might be able to address.

First if you make a call to QDesktopWidget::screenNumber(..) in an objects constructor it will always return 0 even if the object is not on screen 0; at least with Qt3. I have not tested this with Qt4. You must make this call only after the dialog is initialized to get a correct result on a multi-screen system.

Second this does work on an Xinerama machine in that numScreens will return 2 on a dual screen setup and screenNumber() will return the correct result if it is not called in the constructor for the object (IE. you will get a 1 if you are on the second display).

And lastly Qt is xinerama aware if it is configured correctly when it is built. If you look in the Qt code that implements the QDesktopWidget functionality , I looked at this code when figuring out how QDesktopWidget::screenNumber() worked, you will see that it has conditional compilation setup to do things differently for a xinerama aware build. So perhaps the systems in question do not have a correctly configured Qt installation?

[high_flyer]

Thanks for the input!
I will look this up at next opportunity and will report.