I typically work in one project per tmux session. When I create a given tmux session, the default directory is for that project. All new windows and pane splits will open at that default directory. This generally is the default behavior I want.
One caveat: I often open a new window within an existing session that I want anchored to another directory. This could be because I'm working in a monorepo and I need to work from a subdirectory for a specific package or app. Or it could be that I'm temporarily digging into another project and it isn't worth create a whole new session.
Regardless of the reason, I run into a bit of friction with tmux's defaults.
First I open the new window and cd to another project. After some working, I
need to open a split pane, maybe to run a project command like a build or dev
server. Hitting prefix-" (horizontal split) or prefix-% (vertical split)
opens a pane with the shell defaulting back to the original directory, not the
current directory.
The trick to fixing this bit of friction is overriding the directory of pane
splits. I can do that by adding the following to my ~/.tmux.conf:
# Pane splits should open to the same path as the current pane
bind '"' split-window -v -c "#{pane_current_path}"
bind % split-window -h -c "#{pane_current_path}"
Make sure to run tmux source-file ~/.tmux.conf to apply these config changes.
The pane_current_path is called a "Format" in tmux parlance. It resolves to
the absolute path of the current pane's current directory. You can find all the
formats in the manpage with this command: man tmux | less +'/^FORMATS'. You
can also show yourself that this format resolves to what you expect by running
tmux display-message -p '#{pane_current_path}'.