feature(shell): shorten the path in the agnoster prompt, if it gets too long

Reviewed-on: #74

modules/home-manager/shell.nix

@@ -23,6 +23,35 @@
         fi
       '';
     };
+
+    # Unlike oh-my-zsh.extraConfig above, this lands *after*
+    # `source $ZSH/oh-my-zsh.sh`, which is where the theme's prompt functions
+    # come into existence.
+    initContent = lib.mkAfter ''
+      # Keep the prompt on one line in deeply nested trees.
+      #
+      # Both themes render the working directory as `%~`, the full path with
+      # $HOME collapsed to `~`. Somewhere like
+      # ~/git/bfh/cas-pml-sem/documents/bericht that is 39 columns on its own,
+      # and together with user@host and a long branch name the line wraps.
+      #
+      # `%(4~|…/%2~|%~)` is a prompt ternary: at four or more components show
+      # an ellipsis and the last two, otherwise show the path whole. Short
+      # paths like ~/git/nixconfig are left alone.
+      if [[ "$ZSH_THEME" == "agnoster" ]]; then
+        # agnoster calls prompt_dir on every render, so redefining it is
+        # enough. AGNOSTER_DIR_* keep the segment's own colours.
+        prompt_dir() {
+          prompt_segment "$AGNOSTER_DIR_BG" "$AGNOSTER_DIR_FG" '%(4~|…/%2~|%~)'
+        }
+      else
+        # gentoo builds PROMPT once at load time, so patch the string. Its
+        # `%~` is the non-root branch of `%(!.%1~.%~)`, and it is the first
+        # literal `%~` in there, so the root branch is untouched. ASCII `...`
+        # because this branch is the Linux console, whose font may lack U+2026.
+        PROMPT="''${PROMPT/\%~/%(4~|.../%2~|%~)}"
+      fi
+    '';
   };
 
   # set important env vars