Some tooltips won't display in GTK client
When testing https://codereview.tryton.org/58351002/ I cannot get some tooltips to display in the GTK Client.
It only seems to be a problem for the fields that are Many2Many or Many2One fields and are the only field displayed on a page in a notebook.
How to reproduce:
* Apply the patch at https://codereview.tryton.org/58351002/
* Open the Tryton GTK client.
* Open the list of Users and switch to form view.
* The tooltips for the Groups, and Applications will not display, but all other tooltips work (including those for Actions and Employees).