Prevent infinite relayout when adding scrollbars

When one scrollbar is added, this may cause the other to be
needed as well. This change does a second calculation
immediately instead of relying on a signal through a
QueuedConnection.

Task-number: QTBUG-62818
Task-number: QTBUG-56280
Change-Id: Iee9a083e3a6cd3765e6bb9206687a8a6e7f99cff
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

2 files changed, 20 insertions, 6 deletions

diff --git a/src/widgets/widgets/qabstractscrollarea.cpp b/src/widgets/widgets/qabstractscrollarea.cpp
index 249ebd35d3..c6d66bafd4 100644
--- a/src/widgets/widgets/qabstractscrollarea.cpp
+++ b/src/widgets/widgets/qabstractscrollarea.cpp
@@ -334,16 +334,27 @@ void QAbstractScrollAreaPrivate::setSingleFingerPanEnabled(bool on)
 
 void QAbstractScrollAreaPrivate::layoutChildren()
 {
+    bool needH = false;
+    bool needV = false;
+    layoutChildren_helper(&needH, &needV);
+    // Call a second time if one scrollbar was needed and not the other to
+    // check if it needs to readjust accordingly
+    if (needH != needV)
+        layoutChildren_helper(&needH, &needV);
+}
+
+void QAbstractScrollAreaPrivate::layoutChildren_helper(bool *needHorizontalScrollbar, bool *needVerticalScrollbar)
+{
     Q_Q(QAbstractScrollArea);
     bool htransient = hbar->style()->styleHint(QStyle::SH_ScrollBar_Transient, 0, hbar);
-    bool needh = (hbarpolicy != Qt::ScrollBarAlwaysOff) && ((hbarpolicy == Qt::ScrollBarAlwaysOn && !htransient)
-                 || ((hbarpolicy == Qt::ScrollBarAsNeeded || htransient)
-                     && hbar->minimum() < hbar->maximum() && !hbar->sizeHint().isEmpty()));
+    bool needh = *needHorizontalScrollbar || ((hbarpolicy != Qt::ScrollBarAlwaysOff) && ((hbarpolicy == Qt::ScrollBarAlwaysOn && !htransient)
+                            || ((hbarpolicy == Qt::ScrollBarAsNeeded || htransient)
+                            && hbar->minimum() < hbar->maximum() && !hbar->sizeHint().isEmpty())));
 
     bool vtransient = vbar->style()->styleHint(QStyle::SH_ScrollBar_Transient, 0, vbar);
-    bool needv = (vbarpolicy != Qt::ScrollBarAlwaysOff) && ((vbarpolicy == Qt::ScrollBarAlwaysOn && !vtransient)
-                 || ((vbarpolicy == Qt::ScrollBarAsNeeded || vtransient)
-                     && vbar->minimum() < vbar->maximum() && !vbar->sizeHint().isEmpty()));
+    bool needv = *needVerticalScrollbar || ((vbarpolicy != Qt::ScrollBarAlwaysOff) && ((vbarpolicy == Qt::ScrollBarAlwaysOn && !vtransient)
+                            || ((vbarpolicy == Qt::ScrollBarAsNeeded || vtransient)
+                            && vbar->minimum() < vbar->maximum() && !vbar->sizeHint().isEmpty())));
 
     QStyleOption opt(0);
     opt.init(q);
@@ -522,6 +533,8 @@ void QAbstractScrollAreaPrivate::layoutChildren()
         viewportRect.adjust(left, top, -right, -bottom);
 
     viewport->setGeometry(QStyle::visualRect(opt.direction, opt.rect, viewportRect)); // resize the viewport last
+    *needHorizontalScrollbar = needh;
+    *needVerticalScrollbar = needv;
 }
 
 /*!
diff --git a/src/widgets/widgets/qabstractscrollarea_p.h b/src/widgets/widgets/qabstractscrollarea_p.h
index c52e7f9fd4..49f97bcb61 100644
--- a/src/widgets/widgets/qabstractscrollarea_p.h
+++ b/src/widgets/widgets/qabstractscrollarea_p.h
@@ -95,6 +95,7 @@ public:
 
     void init();
     void layoutChildren();
+    void layoutChildren_helper(bool *needHorizontalScrollbar, bool *needVerticalScrollbar);
     // ### Fix for 4.4, talk to Bjoern E or Girish.
     virtual void scrollBarPolicyChanged(Qt::Orientation, Qt::ScrollBarPolicy) {}
     bool canStartScrollingAt( const QPoint &startPos );