Compensate for different rounding rule in CG engine

After we switched to fractional metrics, Core Graphics engine
always put text at floored vertical postion, while underlines
will be rounded after 0.75. Ceiling the underline position will
make sure it will always leave the underline pixels given.

Task-number: QTBUG-19959
Reviewed-by: Eskil

1 files changed, 8 insertions, 2 deletions

diff --git a/src/gui/painting/qpainter.cpp b/src/gui/painting/qpainter.cpp
index 604c1ab..754b16e 100644
--- a/src/gui/painting/qpainter.cpp
+++ b/src/gui/painting/qpainter.cpp
@@ -6335,10 +6335,16 @@ static void drawTextItemDecoration(QPainter *painter, const QPointF &pos, const
 
     QLineF line(pos.x(), pos.y(), pos.x() + width, pos.y());
 
-    const qreal underlineOffset = fe->underlinePosition().toReal();
+    qreal underlineOffset = fe->underlinePosition().toReal();
+    qreal y = pos.y();
+    // compensate for different rounding rule in Core Graphics paint engine,
+    // ideally code like this should be moved to respective engines.
+    if (painter->paintEngine()->type() == QPaintEngine::CoreGraphics) {
+        y = qCeil(y);
+    }
     // deliberately ceil the offset to avoid the underline coming too close to
     // the text above it.
-    const qreal underlinePos = pos.y() + qCeil(underlineOffset);
+    const qreal underlinePos = y + qCeil(underlineOffset);
 
     if (underlineStyle == QTextCharFormat::SpellCheckUnderline) {
         underlineStyle = QTextCharFormat::UnderlineStyle(QApplication::style()->styleHint(QStyle::SH_SpellCheckUnderlineStyle));