New command /layout has been added and a new option weechat.look.save_layout_on_exit. WeeChat now saves layout (windows layout and order of buffers). With /layout command, you can show saved layout, apply layout, and save layout for windows and/or buffers.
By default, WeeChat automatically saves layout when exiting (by /quit or /upgrade), and restore it when starting.

Windows layout is position of windows (including size), and buffer displayed in windows (plugin and buffer name).
Buffers layout is order of buffers, ie numbers assigned to buffers.