I've been testing the new mobile chat, and I've found that the text entry box is too small, which makes writing longer messages difficult.
It's only large enough to show one line, so if my message spans more than one line, the other lines are cut off completely. The actual text area also does not fill the entire space at the bottom of the screen, leaving a significant number of pixels unused. Slightly expanding the text box vertically would significantly improve its usability.
(Firefox on Android, using both a Nexus 5X and Nexus 9.)