Problem with focus

Hello,
As you can see in the attached test case, we have three controls in the same form. Two xform:input controls and an input type=button html control.
1. Change the value of the first name xf:input (transform "Toulouse" by "Toulous" for example), then click on the button. The both controls have the focus. The background-color of the xf:input.value normally change with constraint attribute.
2. Add the "e" at the end of "Toulous" and click to the button. The background-color of the xf:input.value doesn't change and the value contained in the instance doesn't change too.
3. Click in the surname control. The first name control seems not lost the focus.
4. Click in the first name control and next click in the surname control, the firstname control lost focus, the value contained in the instance changes.
I hope all is clear. Sorry if not.
Regards
Frédéric