Here's some code I use to expand the tree all the way to the root.

It's a recursive function that accepts a QTreeWidgetItem *item pointer and travels up the tree, starting as position of *item, and expands all parents in the *item's branch until it reaches the *item's topLevelItem.

Qt Code: 
  1. void MyWidget::expandTree(QTreeWidgetItem *item) {
  2.     QTreeWidgetItem *parent = item->parent();
  3.     if (parent) {
  4.        parent->setExpanded(true);
  5.        int topLevel = table->indexOfTopLevelItem(parent);
  6.        if (topLevel!=-1) {
  7.         QTreeWidgetItem *root = table->topLevelItem(topLevel);
  8.         root->setExpanded(true);
  9.        } else {
  10.            if (item->parent()) {
  11.                expandTree(item->parent());
  12.            }
  13.        }
  14.     }
  15. }
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