Revert "Emit updateBlock signal in QTextDocumentLayout"

This reverts commit 13040043b2f2274de74c296ede7dd3e6ffb67dcb.
It introduced a bad regression, noticeable for longer documents, as
it would cause the documentChanged(0, length) to trigger a layout of
the entire document.

The bug report for the commit (or the commit itself) does not contain
a test case, but it is regardless the wrong approach. Note that
QQuickTextEdit already listens to the contentsChange signal and
invalidates the changed parts of the document as a reaction to this,
so it should already work as expected.

[ChangeLog][Qt Gui][Text] Fixed performance hit from showing large
QTextDocuments in a QTextEdit or QTextBrowser. (Regression introduced
in Qt 5.3.0)

Task-number: QTBUG-51411
Change-Id: I6e7fbf8f62a1d68779eef5da3781de14d9fdcad8
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 56 insertions, 0 deletions

diff --git a/tests/benchmarks/widgets/widgets/qtextbrowser/main.cpp b/tests/benchmarks/widgets/widgets/qtextbrowser/main.cpp
new file mode 100644
index 0000000000..e469ddcb5b
--- /dev/null
+++ b/tests/benchmarks/widgets/widgets/qtextbrowser/main.cpp
@@ -0,0 +1,56 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QDebug>
+#include <QTextBrowser>
+#include <qtest.h>
+
+class tst_QTextBrowser : public QObject
+{
+    Q_OBJECT
+private slots:
+    void largeDocumentsLazyLayout();
+};
+
+void tst_QTextBrowser::largeDocumentsLazyLayout()
+{
+    QString sl;
+    for (int i = 0; i < 150000; ++i) {
+        sl.append("long long text\n");
+    }
+
+    QBENCHMARK {
+        QTextBrowser browser;
+        browser.setPlainText(sl);
+        browser.show();
+    }
+}
+
+QTEST_MAIN(tst_QTextBrowser)
+
+#include "main.moc"