Add api.depends on computed diff

document_page/models/document_page_history.py

@@ -35,6 +35,7 @@ class DocumentPageHistory(models.Model):
     diff = fields.Text(compute='_compute_diff')
 
     @api.multi
+    @api.depends('content', 'page_id.history_ids')
     def _compute_diff(self):
         """Shows a diff between this version and the previous version"""
         history = self.env['document.page.history']