tmux: Better simulate vim's C-e & C-y

Keep the cursor position or selection while scrolling.

.config/tmux/tmux.conf

@@ -112,6 +112,28 @@ bind -T copy-mode-vi I send -X cancel
 bind -T copy-mode-vi a send -X cancel
 bind -T copy-mode-vi i send -X cancel
 
+# Simulate vim's C-e & C-y (Keep cursor position while scrolling). As tmux
+# cannot scroll past the first and last history line, both bindings stop
+# scrolling when the respective line is visible. This mimics the behaviour of
+# vim's C-y at the top of the file.
+bind -T copy-mode-vi C-e \
+	if -F "#{>:#{scroll_position},0}" {
+		send-keys -X scroll-down
+		if -F "#{>:#{copy_cursor_y},#{scroll_region_upper}}" {
+			send-keys -X cursor-up
+		}
+	}
+
+# TODO: C-y bugs shortly after the start for a couple of lines. I have no idea
+# why, but I think the problem lays in the first if, as nothing happens at all
+bind -T copy-mode-vi C-y \
+	if -F "#{<:#{scroll_position},#{history_size}}" {
+		send-keys -X scroll-up
+		if -F "#{<:#{copy_cursor_y},#{scroll_region_lower}}" {
+			send-keys -X cursor-down
+		}
+	}
+
 # Navigate panes with <prefix>-[hjkl]
 # NOTE: C-[hjkl] (w/o prefix) moves through vim splits and tmux panes
 #       See vim-tmux-navigator