Page spread view is weird when viewport width/height is specified

[MurakamiShinyu]

Test sample HTML: https://output.jsbin.com/lifizum

In this sample, <meta name="viewport" content="width=1200, height=1536"> is specified in HTML.

With Vivliostyle viewer:
https://vivliostyle.github.io/vivliostyle.js/viewer/vivliostyle-viewer.html#x=https://output.jsbin.com/lifizum

When the page view mode is "single page" the result is good, but when the page view mode is "spread", the result is weird, i.e., the page spread is offset to bottom and page width is incorrect. See the screenshot:

[MurakamiShinyu]

-epubx-viewport { width; height; } has same problem.

[MurakamiShinyu]

Fixed.