Fix performance regression when avoiding scrollbar flipping

Amends 6c4dc722cb9bf765904feefff4fb00bdb0b3dc9f.

Don't search for the optimal size of the scrollarea's widget if it can't
be found anyway. Try the size with scrollbar first, which covers the
vast majority of sizes.

Optimizing the loop with e.g. a binary search adds no value, as the size
is often just a pixel too small.

Since we can't rely on the number of height-for-width calls, we can't
meaningfully test this behavior. The number of calls is still very high
during showing and resizing; optimizing this further is for a separate
commit.

Fixes: QTBUG-97811
Change-Id: If145302e6414b32cf1ce7251ff33b0039f584867
Reviewed-by: Jonas Kvinge <jonas@jkvinge.net>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit ffc9323671d045e3566980d9ed4567f071004e65)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/widgets/widgets/qscrollarea.cpp b/src/widgets/widgets/qscrollarea.cpp
index f880240ea7..e8fdadb648 100644
--- a/src/widgets/widgets/qscrollarea.cpp
+++ b/src/widgets/widgets/qscrollarea.cpp
@@ -203,10 +203,13 @@ void QScrollAreaPrivate::updateScrollBars()
             if (vbarpolicy == Qt::ScrollBarAsNeeded) {
                 int vbarWidth = vbar->sizeHint().width();
                 QSize m_hfw = m.expandedTo(min).boundedTo(max);
-                while (h > m.height() && vbarWidth) {
-                    --vbarWidth;
-                    --m_hfw.rwidth();
-                    h = widget->heightForWidth(m_hfw.width());
+                // is there any point in searching?
+                if (widget->heightForWidth(m_hfw.width() - vbarWidth) <= m.height()) {
+                    while (h > m.height() && vbarWidth) {
+                        --vbarWidth;
+                        --m_hfw.rwidth();
+                        h = widget->heightForWidth(m_hfw.width());
+                    }
                 }
                 max = QSize(m_hfw.width(), qMax(m_hfw.height(), h));
             }