Add 'visiblelines' setting for 'chatbox' - control visible space on scrollback

src/main/resources/configuration.txt

@@ -90,6 +90,8 @@ components:
     messagettl: 5
     # Optional: set number of lines in scrollable message history: if set, messagettl is not used to age out messages
     #scrollback: 100
+    # Optiona; set maximum number of lines visible for chatbox
+    @visiblelines: 10
     # Optional: send push button
     sendbutton: false
   - class: org.dynmap.ClientComponent