Blinking scrollbar with QScrollArea and edge cases.

#include <QApplication>
#include <QScrollArea>
#include <QWidget>
class Square : public QWidget {
 public:
  int heightForWidth(int width) const override { return width; }
};
int main(int argc, char **argv) {
  QApplication app(argc, argv);
  Square *square = new Square();
  QSizePolicy policy = square->sizePolicy();
  policy.setHeightForWidth(true);
  square->setSizePolicy(policy);
  QScrollArea *area = new QScrollArea();
  area->setWidget(square);
  area->setWidgetResizable(true);
  area->resize(501, 499);
  area->show();
  return app.exec();
}

diff --git a/src/widgets/widgets/qscrollarea.cpp b/src/widgets/widgets/qscrollarea.cpp
--- a/src/widgets/widgets/qscrollarea.cpp
+++ b/src/widgets/widgets/qscrollarea.cpp
@@ -194,16 +194,35 @@ void QScrollAreaPrivate::updateScrollBar
 
     if (resizable) {
         if ((widget->layout() ? widget->layout()->hasHeightForWidth() : widget->sizePolicy().hasHeightForWidth())) {
-            QSize p_hfw = p.expandedTo(min).boundedTo(max);
-            int h = widget->heightForWidth( p_hfw.width() );
-            min = QSize(p_hfw.width(), qMax(p_hfw.height(), h));
+            QSize hfw = m.expandedTo(min).boundedTo(max);
+            QSize min2 = QSize(hfw.width(), qMax(hfw.height(), widget->heightForWidth(hfw.width())));
+
+            if (m.expandedTo(min2) == m && m.boundedTo(max) == m) {
+                p = m; // no scroll bars needed
+                min = min2;
+            } else {
+                hfw = p.expandedTo(min).boundedTo(max);
+                min2 = QSize(hfw.width(), qMax(hfw.height(), widget->heightForWidth(hfw.width())));
+
+                if (vbarpolicy != Qt::ScrollBarAsNeeded
+                    || p.width() <= m.width() - vbar->sizeHint().width()
+                    || min2.boundedTo(max).height() <= p.height()) {
+                    min = min2;
+                } else {
+                    p.setWidth(m.width() - vbar->sizeHint().width());
+                    hfw = p.expandedTo(min).boundedTo(max);
+                    min = QSize(hfw.width(), qMax(hfw.height(), widget->heightForWidth(hfw.width())));
+                }
+            }
+        } else {
+            if (m.expandedTo(min) == m && m.boundedTo(max) == m)
+                p = m; // no scroll bars needed
         }
+    } else {
+        if (m.expandedTo(widget->size()) == m)
+            p = m; // no scroll bars needed
     }
 
-    if ((resizable && m.expandedTo(min) == m && m.boundedTo(max) == m)
-        || (!resizable && m.expandedTo(widget->size()) == m))
-        p = m; // no scroll bars needed
-
     if (resizable)
         widget->resize(p.expandedTo(min).boundedTo(max));
     QSize v = widget->size();