resizing a QTreeWidget

[wysota]

Is there any way to ask Qt about these numbers?

"2" for the spacing between columns

Probably not.

"16" for the width of a scrollbar

verticalScrollbar()->width() should do the trick. Just check if the scrollbar should be visible at all.

[drhex]

Just check if the scrollbar should be visible at all.

...by knowing how many item one wants to show and how much space each
of them will use up and how much space there is available, I presume.

Qt doesn't seem to know [whether a scrollbar is needed] by the time sizeHint is called;
The scrollbar width() always returns 100 there, it changes into 16 once it is actually rendered on screen.

[wysota]

...by knowing how many item one wants to show and how much space each
of them will use up and how much space there is available, I presume.

I don't know is such assumptions are appropriate. Calling verticalScrollBar()->isVisible() might be more reliable.

Qt doesn't seem to know [whether a scrollbar is needed] by the time sizeHint is called;

That doesn't matter. You can change the size hint once the widget is rendered for the first time using QWidget::updateGeometry().

The scrollbar width() always returns 100 there, it changes into 16 once it is actually rendered on screen.

Yes, that's normal for every widget.

[drhex]

Just found a way to get the size of the scrollbar without having to display one first

qApp->style()->pixelMetric(QStyle::PM_ScrollBarExtent)

qApp->style()->pixelMetric(QStyle::PM_ScrollBarExtent)

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