Emulate mouse scrolling for alternate buffers

For pagers such as `less` which don't support terminal mouse features
enable scrolling with the mouse wheel by emulating the behaviour using
`send-keys -t= {up,down}` when in alternate (fullscreen) mode.

These bindings retain the expected behaviour when using programs which
do support terminal mouse events and copy-mode.

tmux.conf

@@ -80,6 +80,32 @@ bind C-u copy-mode -u
 # Begin selection in copy mode with v not Space
 bind -T copy-mode-vi v send-keys -X begin-selection
 
+# Enable mouse scrolling up in alternate buffers.
+# If #{mouse_any_flag} is 1 the pane is running with mouse support, e.g. vim.
+# If #{alternate_on} is 1 the terminal is in alternate buffer mode, e.g. less.
+# If #{pane_in_mode} is 1 the pane is in copy-mode.
+bind -n WheelUpPane \
+  if -Ft= "#{mouse_any_flag}" \
+    "send-keys -M" \
+    "if -Ft= \"#{alternate_on}\" \
+      \"send-keys -t= Up ; \
+        send-keys -t= Up ; \
+        send-keys -t= Up ; \" \
+      \"select-pane -t= ; \
+        if -Ft= '#{pane_in_mode}' \
+          'send-keys -M' \
+          'copy-mode -e ; send-keys -M' \" "
+# Emulate mouse scrolling down in alternate buffers.
+bind -n WheelDownPane \
+  if -Ft= "#{mouse_any_flag}" \
+    "send-keys -M" \
+    "if -Ft= \"#{alternate_on}\" \
+      \"send-keys -t= Down ; \
+        send-keys -t= Down ; \
+        send-keys -t= Down ; \" \
+      \"select-pane -t= ; \
+        send-keys -M\" "
+
 # Yank to the system clipboard in copy mode
 if -b '[ "`uname`" = "Darwin" ]' \
   'bind -T copy-mode-vi y send-keys -X copy-pipe-and-cancel \