File dialog detailed view columns were made fixed-size in commit #e504bc1fd56412ee7e9748a0dfafa537977ec1b5, leading to bug 401506.
This patch restores the interactive mode users expect in detailed file list views but preserves the "ideal" column sizing.
Qt doesn't allow combining modes (QHeaderView::Stretch|QHeaderView::Interactive) so a bit of a ruse is required to activate interactive mode after the definitive sizes have been set. This patch achieves that as follows:
1 A slot is connected to QHeaderView::sectionResized when a Polish event is received, and state member variables are initialised.
2 The handler (slot) stores new, positive sizes for individual columns (sections) under condition that the QTreeView contains entries. No attempt i made to change the QHeaderView settings directly from here. All this happens before the widget will be drawn (evidently(?)).
3 The state information is checked just before the widget is to be drawn (when the 1st Paint event is received); if sizing information is available the corresponding columns are resized to their respective stored sizes explicitly, and the entire QHeaderView is set to interactive resize mode.
In my testing the first paint event always comes after the final column sizes have been determined, and it is not too late to change resize mode here.
Currently the handler slot from 1) is a lambda insteead of a member function. A member function could be disconnected once the required information is obtained and acted upon but handler overhead is negligible and file dialogs are typically not long-lived.
Possible and welcome future development: save and restore user-defined column sizes. The resize handler would have to remain in place for that (and track new sizes signalled when resizeMode==Interactive).