tmux: Manually "backport" scroll-middle if missing

First working draft

.config/tmux/tmux.conf

@@ -227,6 +227,24 @@ bind -T copy-mode-vi C-y {
 	}
 }
 
+# Scroll so that the current line becomes the middle one (vim's zz)
+if -F "#{<:#{version},3.4}" {
+	# Simulate when scroll-middle does not exist yet
+	# TODO: do the scroll_position and cursor_y calculations only once here
+	bind -T copy-mode-vi z {
+		set -F @middle_line "#{e|/:#{e|-:#{pane_height},1},2}"
+		set -F @scroll_to_middle "#{e|-:#{copy_cursor_y},#{@middle_line}}"
+		if -F "#{e|<:#{@scroll_to_middle},0}" {
+			run "tmux send -N #{e|*:-1,#{@scroll_to_middle}} C-y"
+		} { if -F "#{e|>:#{@scroll_to_middle},0}" {
+			run "tmux send -N #{@scroll_to_middle} C-e"
+		} }
+	}
+} {
+	# should be the default, but here for verboseness
+	bind -T copy-mode-vi z send -X scroll-middle
+}
+
 # Navigate panes with <prefix>-[hjkl]
 # NOTE: C-[hjkl] (w/o prefix) moves through vim splits and tmux panes
 #       See vim-tmux-navigator