Mano a mano with Gtk scrollbars again

So far it's been a losing battle, but perhaps this time? The last change
to get rid of the pesky Gtk warning resulted in erratic scroll behaviour
instead due to this Gtk issue:

https://bugzilla.gnome.org/show_bug.cgi?id=765410

Apparently we cannot get syncing of scrollbars to work at all from within
the size allocate handler. Since none of the other queueing mechanisms
seems to have an effect (queue_resize, etc.) we just bend over and add
an ugly work-around based on triggering an additional refresh after the
size allocate is done.

For issue howl-editor#371.

lib/aullar/view.moon

@@ -4,6 +4,7 @@
 ffi = require 'ffi'
 bit = require 'bit'
 ffi_cast = ffi.cast
+callbacks = require 'ljglibs.callbacks'
 
 Gdk = require 'ljglibs.gdk'
 Gtk = require 'ljglibs.gtk'
@@ -21,7 +22,8 @@ config = require 'aullar.config'
 {:define_class} = require 'aullar.util'
 {:parse_key_event} = require 'ljglibs.util'
 {:max, :min, :floor} = math
-
+cast_arg = callbacks.cast_arg
+C = ffi.C
 append = table.insert
 
 jit.off true, true
@@ -148,6 +150,9 @@ View = {
     @buffer = buffer
     @config\add_listener self\_on_config_changed
 
+    @_refresh_cb_handle = callbacks.register self\_sync_scrollbars, "view-#{@}-refresh"
+    @_cb_handler = callbacks.unref_handle @_refresh_cb_handle
+
   destroy: =>
     @bin\destroy!
 
@@ -825,24 +830,34 @@ View = {
 
   _on_size_allocate: (_, allocation) =>
     allocation = ffi_cast('GdkRectangle *', allocation)
+    resized = (not @height or @height != allocation.height) or
+      (not @width or @width != allocation.width)
+    return unless resized
+
     gdk_window = @area.window
     @im_context.client_window = gdk_window
     if gdk_window != nil
       gdk_window.cursor = @_cur_mouse_cursor
 
-    is_growing = @height and allocation.height > @height
+    getting_taller = @height and allocation.height > @height
     @width = allocation.width
     @height = allocation.height
     @_reset_display!
 
-    if is_growing and @last_visible_line == @buffer.nr_lines
+    if getting_taller and @last_visible_line == @buffer.nr_lines
       -- since we're growing this could be wrong, and we might need
       -- to re-calculate what the last visible line actually is
       @last_visible_line = @last_visible_line
 
-    @_sync_scrollbars!
     @buffer\ensure_styled_to line: @last_visible_line + 1
 
+    -- we just can't sync the scrollbars here due to a Gtk issue, so we
+    -- work around this by syncing these immediately after the size_allocate
+    -- handling is done
+    -- Ref: https://bugzilla.gnome.org/show_bug.cgi?id=765410
+    C.g_timeout_add_full C.G_PRIORITY_LOW, 0, callbacks.source_func,
+      cast_arg(@_refresh_cb_handle.id), nil
+
   _on_config_changed: (option, val, old_val) =>
     if option == 'view_font_name' or option == 'view_font_size'
       @area\override_font Pango.FontDescription {