Odd behavior of editable tree in One2Many widget

We have found strange behavior of editable tree in One2Many widget (Tryton Client 5.4.8 for Windows).
This effect occurs if number of records exceeds number of visible rows in the widget.

Let’s take Contact Mechanism widget from Party module:
Clipboard01
Now add a new record:
Clipboard02
After pressing the Tab key, we have:
Clipboard03
After the next Tab key is pressed:
Clipboard04
And only after repeatedly pressing the Tab key (or scrolling down to the last row):
Clipboard05
Why the input frame is not on the last (active) row right after adding a new record?