Specify the simulated device viewport

[asajeffrey]

Over in web-platform-tests/wpt#18004, a test that checks that the test device viewport matches the requested resolution was reverted:

-        assert_equals(viewport.width, params.resolution.width);
-        assert_equals(viewport.height, params.resolution.height);
+        assert_greater_than_equal(viewport.width, 1);
+        assert_greater_than_equal(viewport.height, 1);

The reason for reverting it is the spec text:

// This represents the native resolution of the device, but may not reflect the viewport exposed to the page.
// https://immersive-web.github.io/webxr/#xrviewport
dictionary FakeXRDeviceResolution {
    required long width;
    required long height;
};

Can this be changed to specify that the requested resolution should be the resolution of the viewport?

The reason for wanting this is to catch a WebXR implementation bug, where the framebuffer provided to WebXR has a viewport sized to the canvas, rather than for the device (Servo had this bug until recently, which is why I added this test).

[alcooper91]

I’m unable to find anything in the WebXR spec calling out any requirements for sizing the canvas/viewport, so the output size is largely up to the UAs. While the device has a native resolution the UA can decide to scale it up or down based on what they think is the best resolution (potentially modulo memory or other user settings). Something like this would actually be a restriction on the spec implementation and not something that the test api should be able to specify.
If you can find spec text to back either of those restrictions, let me know.

[asajeffrey]

Indeed, the WebXR spec specifies the native framebuffer resolution https://immersive-web.github.io/webxr/#native-webgl-framebuffer-resolution, but not a native viewport size. I'll open an issue on the WebXR spec about this.

[asajeffrey]

Filed immersive-web/webxr#777