The mobile version of chat by default scrolls the page down to the last message every few seconds. There is an option to disable that behaviour, but that setting is reset every time you leave and reenter a room.
I always switch this setting off, as it prevents me from reading the older messages in chat. It is somewhat annoying though that I have to set it everytime I enter a new chat room. I'm usually browsing chat from my favorite rooms list and enter chat when I see that there are recent messages in there. This means I change rooms quite frequently, and every time the option is set back to default.
I propose that the option "stay on bottom" - "don't stay on bottom" is remembered permanently until changed for a logged in user. This would make the mobile chat more convenient to use for me.