WindowStartupLocation DPI-awareness

[OronDF343]

Fixes #1961

Window.SetWindowStartupLocation() will now respect the display scaling when positioning the window. I am using ITopLevelImpl.Scaling to do this.

CenterScreen works for me at 150% on desktop Win 7. CenterOwner should also work in theory - only limited testing was done as I have no more time to work on this today. The code is pretty simple anyways.

[danwalmsley]

unfortunately this doesn't work for me.

[OronDF343]

I believe this is because the ClientSize is being treated as screen pixels somewhere and isn't being scaled. This is probably a backend-specific bug as I'm not getting it on Win32.

Which backend are you running on?

[grokys]

@danwalmsley any further info here?

[ahopper]

I notitce that window position does not seem to use scaling on win32

Avalonia/src/Windows/Avalonia.Win32/WindowImpl.cs

Line 360 in 3d4fb2c

public Point Position

[OronDF343]

Correct, this is why the original issue exists. The question is, is the same true for the other platforms? Is Windows the exception rather than the norm, or vice versa?

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[ahopper]

Yes, it is not clear what should be scaled and what should allow for window chrome and where the transitions should happen.

[grokys]

Yes, I think the problem here is that we're not specific as to whether a window's position is in device co-ordinates or in device independent co-ordinates.

This SO answer suggests that win32's window position will be in device coordinates if you have a DPI-aware app (we do):

In particular, if you call GetWindowRect or GetClientRect from a high-DPI aware application, you will get the actual values in screen coordinates.

My feeling is that this makes sense, and that we should specify that Window.Position is always in device pixels. @kekekeks does that make sense for other platforms?

[kekekeks]

It makes perfect sense to have Window.Position in device pixels. There would be a problem with GTK which scales everything, but we are going to get rid of it anyway.

We should also make Position to use PixelPoint instead of Point and make screens API to use PixelSize. Also, we need to fix public static PixelSize FromSize(Size, double) overload to use scaling factor instead of "dpi" which isn't a DPI anyway.

[grokys]

Thanks for this @OronDF343: the changes @kekekeks suggested made sense, but they were a bit more involved so we merged #2250 which incorporates your changes there.