diff --git a/src/pyj/read_book/iframe.pyj b/src/pyj/read_book/iframe.pyj
index 54a0610792..a5bef026bc 100644
--- a/src/pyj/read_book/iframe.pyj
+++ b/src/pyj/read_book/iframe.pyj
@@ -113,6 +113,7 @@ class IframeBoss:
         handlers = {
             'change_color_scheme': self.change_color_scheme,
             'change_font_size': self.change_font_size,
+            'viewer_font_size_changed': self.viewer_font_size_changed,
             'change_number_of_columns': self.change_number_of_columns,
             'number_of_columns_changed': self.number_of_columns_changed,
             'change_scroll_speed': self.change_scroll_speed,
@@ -317,6 +318,9 @@ class IframeBoss:
                 # apply_font_size() is a no-op
                 self.relayout_on_font_size_change()
 
+    def viewer_font_size_changed(self, data):
+        self.relayout_on_font_size_change()
+
     def change_number_of_columns(self, data):
         if current_layout_mode() is 'flow':
             self.send_message('error', title=_('In flow mode'), msg=_(