Qt Designer inflexibility and assertiveness - criminal level
I made a custom widget subclassing QScrollArea. In its constructor it initialized a persistent viewport widget and and accessed it via a pointer through its work. The form this widget is on is built from .ui file, so, naturally, I promoted QScrollArea. And my program started crashing. It took some time to understand the cause. Qt Designer forced a viewport widget creation even for my promoted widget. Even though my viewport widget was properly initialized in constructor, m_ui.setupUi() of the form created another widget and put it into viewport, destroying my own.
And here comes the point. Viewport widget can not be deleted or have its type changed in Qt Designer. And fiddling with .ui file in text editor has shown that both can actually be done, and Qt Designer works just fine with such a tweaked file. I say Qt Designer needs to be improved to give user more power over child widgets.
To discuss this kind of matter you should rather go on the interest mailing list. You'll find there Qt's developers/maintainers (this forum is more user oriented)
I have actually created an issue in JIRA.
You should share the link so if other are looking for something similar they may find it more easily