Don't set -x/-y in sessions if inside tmux client

Related:

- tmux-python/tmuxp#312
- tmux-python/tmuxp#309

Author: tony

libtmux/server.py

@@ -477,7 +477,9 @@ def new_session(self,
         if start_directory:
             tmux_args += ('-c', start_directory)
 
-        if has_gte_version('2.6'):
+        # tmux 2.6 gives unattached sessions a tiny default area
+        # no need send in -x/-y if they're in a client already, though
+        if has_gte_version('2.6') and 'TMUX' not in os.environ:
             tmux_args += ('-x', 800, '-y', 600)
 
         proc = self.cmd(