Revert "Emit updateBlock signal in QTextDocumentLayout"

This reverts commit 13040043b2.
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>

src/gui/text/qtextdocumentlayout.cpp

@@ -2921,11 +2921,11 @@ void QTextDocumentLayout::documentChanged(int from, int oldLength, int length)
 {
     Q_D(QTextDocumentLayout);
 
-    QTextBlock startIt = document()->findBlock(from);
+    QTextBlock blockIt = document()->findBlock(from);
     QTextBlock endIt = document()->findBlock(qMax(0, from + length - 1));
     if (endIt.isValid())
         endIt = endIt.next();
-    for (QTextBlock blockIt = startIt; blockIt.isValid() && blockIt != endIt; blockIt = blockIt.next())
+     for (; blockIt.isValid() && blockIt != endIt; blockIt = blockIt.next())
          blockIt.clearLayout();
 
     if (d->docPrivate->pageSize.isNull())
@@ -2967,9 +2967,6 @@ void QTextDocumentLayout::documentChanged(int from, int oldLength, int length)
 
     d->insideDocumentChange = false;
 
-    for (QTextBlock blockIt = startIt; blockIt.isValid() && blockIt != endIt; blockIt = blockIt.next())
-         emit updateBlock(blockIt);
-
     if (d->showLayoutProgress) {
         const QSizeF newSize = dynamicDocumentSize();
         if (newSize != d->lastReportedSize) {

tests/benchmarks/benchmarks.pro

@@ -7,6 +7,7 @@ SUBDIRS = \
 qtHaveModule(dbus): SUBDIRS += dbus
 qtHaveModule(network): SUBDIRS += network
 qtHaveModule(gui): SUBDIRS += gui
+qtHaveModule(widgets): SUBDIRS += widgets
 
 check-trusted.CONFIG += recursive
 QMAKE_EXTRA_TARGETS += check-trusted

tests/benchmarks/widgets/widgets.pro

@@ -0,0 +1,3 @@
+TEMPLATE = subdirs
+SUBDIRS = \
+        widgets \

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"

tests/benchmarks/widgets/widgets/qtextbrowser/qtextbrowser.pro

@@ -0,0 +1,6 @@
+QT += testlib widgets
+
+TEMPLATE = app
+TARGET = tst_bench_QTextBrowser
+
+SOURCES += main.cpp

tests/benchmarks/widgets/widgets/widgets.pro

@@ -0,0 +1,3 @@
+TEMPLATE = subdirs
+SUBDIRS = \
+        qtextbrowser \