You didn't answer the question - does it still reproduce with letterbox scale (not the integer version)?
400x240 doesn't fit exactly in to a 1920x1080 display, so the black borders are to be expected in that case. On the other hand if the resolution does fit that exactly, then there should be no borders. So to figure out which one is the bug, what is the resolution of the device you are testing on?
Also one thing you can do to find out more: display the Browser plugin's WindowInnerWidth and WindowInnerHeight expressions on the screen and see what they say in both cases. If they change, it's probably a browser bug, and is caused by the browser telling C2 the wrong window size. On the other hand if they're always the same even in both cases, it's probably a C2 bug. Could you give that a shot?