[Gambas-user] Normal behaviour for GTK+?

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This has me pulling my hair out. I think I have narrowed the problem
down to the GTK+ toolkit for Gambas.

If I create a GTK+ project and put a TextBox and a ValueBox on a form
and set their default values, when I tab to the TextBox I can over-type
the text, but when I tab to the ValueBox, the number I type is appended:

For example, if the ValueBox contains 5000 and I type 7, the value
becomes 50007.

However if I change the project to use a Qt GUI, it works as I expect,
i.e. if the ValueBox contains 5000 and I type 7, the value becomes 7. I
have been able to reproduce this behaviour.

Is that normal for GTK+? Is there anything I can do short of using Qt?
Am I doing something wrong?

Also, I can't find any documentation why I might use Qt in Gambas as
opposed to GTK+ in Gambas. Opinions on what is best and why will be
appreciated.