qml: control bar delayed calculation should be done at appropriate time
The calculation is not necessary, when when
is not set. So, there is
no need to do the calculation.
At the same time, the calculation must be done after when
is set.
Currently, there is no guarantee for that and it causes issues with
expanding items getting invisible unexpectedly.
Request review @chub.
As a side note, there have been some attempts from Qt's side to change
the type of value
to QQmlScriptString
, but it has never been merged.