Mobile: Fix parts of reply panel out of viewport

Fix not all content in reply panel visible due to panel overflowing on small Androids with keyboard up.

The bug presents itself on Chrome, e.g. on a 800x480 device (reported viewport width 320px), due to the URL bar chrome reappearing as the Reply panel opens (even if the URL bar was hidden before).
Even Chrome@Moto G (1280x720, viewport width 360px) has a bit of overflow.

![replypanelpartsinvisible](https://cloud.githubusercontent.com/assets/5316880/3490872/6b88e3cc-057f-11e4-81ce-65f6934ccefa.png)