--cap "screenResolution=2560x1600"
viewportHeight: 1080,
viewportWidth: 1920,
Notice the resolution is 1279X1023. I think this may be coming from line 12937 of siesta-launcher-all.js and that the width and height may not be set as expected.
Note that the iframe size appears to be set properly since I see 2/3 of the app I'm testing (with vertical and horizonal scrollbars). The VM resolution appears to be set properly as well. This is just and issue with the size of the browser window itself.
Can you confirm?