The mcabber documentation doesn't say how to display a desired chat
buffer.  Joining a room (/room join) creates a chat buffer, the existence
of the buffer can be seen (/buffer list), and some buffer can be shown
by entering chat mode (<enter>), but the documentation doesn't indicate
any way to select *which* buffer is shown in chat mode.  In fact the
"[status]" buffer is initially shown, so there's no documented way to
display an actual room buffer.  I presume there is some way, obviously
undocumented.  That such a critical function is not documented seriously
damages the usability of this program to people who don't already know
how to use it.

The manpage says:
PageUp and PageDown keys are used to move in the roster.

If I change the entity in the roster, the chat buffer will be
updated. With the /info command I see which buffer is displayed.

Do you mean something like:

PageUp and PageDown keys are used to move in the roster. The chat
window will be updated, if you change the entity in your roster.
Details of the current buffer can be shown via /info command.

Let me know if it will solve the problem, I can forward this
request to upstream project.