With the latest version of tmux there was a problem with the -p option causing tmux to return -1 and therefore blocking the splitting.
From the tmux documentation:
The -l option specifies the size of the new pane in lines (for vertical split) or in columns (for horizontal split); size may be followed by ‘%’ to specify a percentage of the available space.
We can use the -l option to do both percentage and line resizing
With the latest version of tmux there was a problem with the -p option causing tmux to return -1 and therefore blocking the splitting. From the tmux documentation:
We can use the
-loption to do both percentage and line resizing