looked at this for a while to no avail. It seems to be another case of one thing rounding up and another rounding down.

The closest I got to figuring anything out was to note that in NativeViewHost when we call native_wrapper_->ShowWidget(), which takes DIP arguments, modifying the values will modify the web contents' display size. This suggests that somewhere along the line it's these DIPs that are getting incorrectly floored --- it seems like there's a flooring operation which should be a rounding operation.