QTreeView: expandToDepth() and expandAll() ends prematurely for asynchronous models

Issue

When the model is asynchronous (like QFileSystemModel, or the example model at https://code.qt.io/cgit/qt/qtopcua.git/tree/examples/opcua/opcuaviewer ), expandToDepth() might stop expanding before the desired depth is reached.

The same occurs with expandAll().

Demo

#include <QtWidgets>

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);

	auto model = new QFileSystemModel;
	model->setRootPath("C:/tmp");

	QTreeView tree;
	tree.setModel(model);

	QPushButton button("Click me to expand");
	QObject::connect(&button, &QPushButton::clicked, [&]{
		tree.expandToDepth(3);
	});

	tree.show();
	button.show();

	return a.exec();
}

• Expected behaviour: 1 click should expand the tree to depth 3
• Actual behaviour: Multiple clicks are required to reach depth 3 (Although depending on your filesystem size, the last click will likely freeze the app)

Workaround

Call `expandToDepth(depth)` repeatedly (with a slight wait between each call) until the desired depth is reached. Each call causes the model to fetch a bit more data, so it allows expansion to proceed a bit further.