The 'stage_id' field is now a stored-editable computed field.
The 'company_id' has been converted to a stored-editable computed field.
The modification of the 'partner_id' field in the onchange method has been
moved and merged into the _compute_partner_id method.
Remove the empty 'onchange' method from the project.task model and also remove
this method call from the sale_timesheet module, which inherits from the project.task model.
Adapt the _compute_partner_id to retrieve exactly the behaviour before the onchange
method deletion.
Update code and tests of the task/subtask partner_id management after discussion with Alexandra to clarify the behavior.
Add the 'multi_edit' attribute to tree views.