xterm 348 and tmux

[Nicholas Marriott]

Hi all

xterm version 348 has changed so that left and right margins are
disabled when the window is resized.

tmux has up to now relied on margins staying enabled until it disables
them. This means that after resizing the window, horizontally split
panes will not draw correctly in tmux.

A fix for this has been made to master and will be in 3.0, it should
also apply to older versions:

https://github.com/tmux/tmux/commit/49bf7dc77e5372a8ec404d1fc2e12b8f68a256d5

The problem can be worked around for older tmux by configuring xterm to
report as VT220 instead of VT420:

xterm -ti 220

Or in .Xresources:

XTerm*decTerminalID: 220

This makes tmux choose not to use margins (which will work but is
slower).