Viewport.getEffectiveBox(): fix for height computation on iOS8. Refs#348

Fixes a tiny code error in Viewport.getEffectiveBox() affecting the height computation on iOS 8 and impacting delite/popup and delite/HasDropDown.

This is a partial fix for #348 - it improves the behavior on iOS 8 with virtual keyboard open, but even with this fix it remains suboptimal (but better than before).