Fix infinite layout loop with negative line width

Setting a negative line width does not make much sense, but in
earlier Qt versions, this work the same as if the line width was 0
(just give you the minimal layout given wrapping constraints).

But since 991c056438, we check if
current width > line width at an earlier point, and because 0 > -1,
we would exit immediately before adding any characters to the text
line.

To restore the behavior in earlier versions, we set the minimum
possible line width to 0.

Pick-to: 6.5
Fixes: QTBUG-109474
Change-Id: Iceadd5135681f61b30de8221853834983941c5a4
Reviewed-by: Lars Knoll <lars@knoll.priv.no>
Reviewed-by: hjk <hjk@qt.io>

src/gui/text/qtextlayout.cpp

@@ -1611,10 +1611,7 @@ void QTextLine::setLineWidth(qreal width)
         return;
     }
 
-    if (width > QFIXED_MAX)
-        width = QFIXED_MAX;
-
-    line.width = QFixed::fromReal(width);
+    line.width = QFixed::fromReal(qBound(0.0, width, qreal(QFIXED_MAX)));
     if (line.length
         && line.textWidth <= line.width
         && line.from + line.length == eng->layoutData->string.size())

tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp

@@ -126,6 +126,7 @@ private slots:
     void softHyphens();
     void min_maximumWidth_data();
     void min_maximumWidth();
+    void negativeLineWidth();
 
 private:
     QFont testFont;
@@ -2715,5 +2716,16 @@ void tst_QTextLayout::min_maximumWidth()
     }
 }
 
+void tst_QTextLayout::negativeLineWidth()
+{
+    QTextLayout layout;
+    layout.setText("Foo bar");
+    layout.beginLayout();
+    QTextLine line = layout.createLine();
+    line.setLineWidth(-1);
+    QVERIFY(line.textLength() > 0);
+    layout.endLayout();
+}
+
 QTEST_MAIN(tst_QTextLayout)
 #include "tst_qtextlayout.moc"