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This topic has been problematic for far too long, and GtkScrolledWindow and GtkViewport have a confusing array of properties yet still fail to do a convincing job.

Outlined solution:

• Add gtk_scrollable_measure_viewport(), with a vfunc in the scrollable interface
• Make GtkScrolledWindow call that instead of consulting its own properties
• Give common scrolled children properties to influence it, like GtkTextView::min-rows