When replacing a widget in a hidden splitter, then we only need to keep
the new widget hidden if the previous widget was hidden.
If the new widget is not explicitly hidden, and the splitter is already
visible, then we need to explicitly show the new widget.
Augment test case; the existing test cases already cover swapping out a
collapsed or hidden widget.
Fixes: QTBUG-102134
Pick-to: 6.3 6.2
Change-Id: I9b60711a5c1cab79777ce4183783114a16ac3394
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
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