We’ve added a keyboard shortcuts plugin (source) to Roundcube. There’s a help popup that shows the shortcuts: use shortcut ? or click the icon of a keyboard on the message list navigation toolbar. Here’s a list of them as well:
Mailbox View
? Help
a Select all visible messages
A Mark all visible messages as read
c Compose
d Delete message
f Forward the message
j Show previous message
k Show next message
p Print this message
r Reply to sender
R Reply to list or to sender and all recipients
s Quick search
u Check for new messages
Threads
E Expand All
C Collapse All
U Expand Unread
Displaying Messages
d Delete message
c Compose
f Forward the message
j Show previous message
k Show next message
p Print this message
r Reply to sender
R Reply to list or to sender and all recipients