Scrollback gets lost upon resizing the window
This is a bug that has been here since 3.8 or maybe even before.
I've show last 20 messages in channel turned on under Logging. When I scroll up in a channel (the window isn't full sized) then make the window full sized I only see the entry from the log of the previous session. Scrolling up and down in full screen to the bottom ends at the last log entry. Making the window smaller again or going with the mouse over the hidden nicklist fixes the issue. The bug is still there when the hide nicklist option is disabled.