Fix decoration rendering with scale factor < 1

QPainter won't let paint with a device pixel ratio less than 1. There
are used to be workarounds to force a device pixel ratio of 1, but they
were removed with fractional scaling corner fix.

This change makes sure that the decoration renderer forces a device
pixel ratio of 1 if the output's scale factor is less than 1 so
calculated texture coordinates match where window borders are rendered
in the texture atlas.

BUG: 449681

src/decorationitem.cpp

@@ -68,6 +68,12 @@ void DecorationRenderer::resetDamage()
     m_damage = QRegion();
 }
 
+qreal DecorationRenderer::effectiveDevicePixelRatio() const
+{
+    // QPainter won't let paint with a device pixel ratio less than 1.
+    return std::max(qreal(1.0), devicePixelRatio());
+}
+
 qreal DecorationRenderer::devicePixelRatio() const
 {
     return m_devicePixelRatio;
@@ -107,7 +113,7 @@ QImage DecorationRenderer::renderToImage(const QRect &geo)
     image.fill(Qt::transparent);
     QPainter p(&image);
     p.setRenderHint(QPainter::Antialiasing);
-    p.setWindow(QRect(geo.topLeft(), geo.size() * qPainterEffectiveDevicePixelRatio(&p)));
+    p.setWindow(QRect(geo.topLeft(), geo.size() * effectiveDevicePixelRatio()));
     p.setClipRect(geo);
     renderToPainter(&p, geo);
     return image;
@@ -230,7 +236,7 @@ WindowQuadList DecorationItem::buildQuads() const
     }
 
     QRect left, top, right, bottom;
-    const qreal devicePixelRatio = m_renderer->devicePixelRatio();
+    const qreal devicePixelRatio = m_renderer->effectiveDevicePixelRatio();
     const int texturePad = DecorationRenderer::TexturePad;
 
     if (const AbstractClient *client = qobject_cast<const AbstractClient *>(m_window)) {

src/decorationitem.h

@@ -38,6 +38,7 @@ public:
     void addDamage(const QRegion &region);
     void resetDamage();
 
+    qreal effectiveDevicePixelRatio() const;
     qreal devicePixelRatio() const;
     void setDevicePixelRatio(qreal dpr);
 

src/scenes/opengl/scene_opengl.cpp

@@ -1536,9 +1536,10 @@ void SceneOpenGLDecorationRenderer::render(const QRegion &region)
     QRect left, top, right, bottom;
     client()->client()->layoutDecorationRects(left, top, right, bottom);
 
-    const int topHeight = std::ceil(top.height() * devicePixelRatio());
-    const int bottomHeight = std::ceil(bottom.height() * devicePixelRatio());
-    const int leftWidth = std::ceil(left.width() * devicePixelRatio());
+    const qreal devicePixelRatio = effectiveDevicePixelRatio();
+    const int topHeight = std::ceil(top.height() * devicePixelRatio);
+    const int bottomHeight = std::ceil(bottom.height() * devicePixelRatio);
+    const int leftWidth = std::ceil(left.width() * devicePixelRatio);
 
     const QPoint topPosition(0, 0);
     const QPoint bottomPosition(0, topPosition.y() + topHeight + (2 * TexturePad));
@@ -1547,14 +1548,15 @@ void SceneOpenGLDecorationRenderer::render(const QRegion &region)
 
     const QRect dirtyRect = region.boundingRect();
 
-    renderPart(top.intersected(dirtyRect), top, topPosition);
-    renderPart(bottom.intersected(dirtyRect), bottom, bottomPosition);
-    renderPart(left.intersected(dirtyRect), left, leftPosition, true);
-    renderPart(right.intersected(dirtyRect), right, rightPosition, true);
+    renderPart(top.intersected(dirtyRect), top, topPosition, devicePixelRatio);
+    renderPart(bottom.intersected(dirtyRect), bottom, bottomPosition, devicePixelRatio);
+    renderPart(left.intersected(dirtyRect), left, leftPosition, devicePixelRatio, true);
+    renderPart(right.intersected(dirtyRect), right, rightPosition, devicePixelRatio, true);
 }
 
 void SceneOpenGLDecorationRenderer::renderPart(const QRect &rect, const QRect &partRect,
-                                               const QPoint &textureOffset, bool rotated)
+                                               const QPoint &textureOffset,
+                                               qreal devicePixelRatio, bool rotated)
 {
     if (!rect.isValid()) {
         return;
@@ -1567,7 +1569,7 @@ void SceneOpenGLDecorationRenderer::renderPart(const QRect &rect, const QRect &p
     int verticalPadding = padding.top() + padding.bottom();
     int horizontalPadding = padding.left() + padding.right();
 
-    QSize imageSize = rect.size() * devicePixelRatio();
+    QSize imageSize = rect.size() * devicePixelRatio;
     if (rotated) {
         imageSize = QSize(imageSize.height(), imageSize.width());
     }
@@ -1575,12 +1577,12 @@ void SceneOpenGLDecorationRenderer::renderPart(const QRect &rect, const QRect &p
     paddedImageSize.rheight() += verticalPadding;
     paddedImageSize.rwidth() += horizontalPadding;
     QImage image(paddedImageSize, QImage::Format_ARGB32_Premultiplied);
-    image.setDevicePixelRatio(devicePixelRatio());
+    image.setDevicePixelRatio(devicePixelRatio);
     image.fill(Qt::transparent);
 
     QRect padClip = QRect(padding.left(), padding.top(), imageSize.width(), imageSize.height());
     QPainter painter(&image);
-    const qreal inverseScale = 1.0 / devicePixelRatio();
+    const qreal inverseScale = 1.0 / devicePixelRatio;
     painter.scale(inverseScale, inverseScale);
     painter.setRenderHint(QPainter::Antialiasing);
     painter.setClipRect(padClip);
@@ -1589,7 +1591,7 @@ void SceneOpenGLDecorationRenderer::renderPart(const QRect &rect, const QRect &p
         painter.translate(0, imageSize.height());
         painter.rotate(-90);
     }
-    painter.scale(devicePixelRatio(), devicePixelRatio());
+    painter.scale(devicePixelRatio, devicePixelRatio);
     painter.translate(-rect.topLeft());
     renderToPainter(&painter, rect);
     painter.end();
@@ -1597,7 +1599,7 @@ void SceneOpenGLDecorationRenderer::renderPart(const QRect &rect, const QRect &p
     // fill padding pixels by copying from the neighbour row
     clamp(image, padClip);
 
-    QPoint dirtyOffset = (rect.topLeft() - partRect.topLeft()) * devicePixelRatio();
+    QPoint dirtyOffset = (rect.topLeft() - partRect.topLeft()) * devicePixelRatio;
     if (padding.top() == 0) {
         dirtyOffset.ry() += TexturePad;
     }
@@ -1641,7 +1643,7 @@ void SceneOpenGLDecorationRenderer::resizeTexture()
                          qMax(left.height(), right.height()));
     size.rheight() = top.height() + bottom.height() +
                      left.width() + right.width();
-    size *= devicePixelRatio();
+    size *= effectiveDevicePixelRatio();
 
     size.rheight() += 4 * (2 * TexturePad);
     size.rwidth() += 2 * TexturePad;

src/scenes/opengl/scene_opengl.h

@@ -213,7 +213,7 @@ public:
     }
 
 private:
-    void renderPart(const QRect &rect, const QRect &partRect, const QPoint &textureOffset, bool rotated = false);
+    void renderPart(const QRect &rect, const QRect &partRect, const QPoint &textureOffset, qreal devicePixelRatio, bool rotated = false);
     static const QMargins texturePadForPart(const QRect &rect, const QRect &partRect);
     void resizeTexture();
     QScopedPointer<GLTexture> m_texture;

src/scenes/qpainter/scene_qpainter.cpp

@@ -444,7 +444,7 @@ void SceneQPainterDecorationRenderer::render(const QRegion &region)
         }
         QPainter painter(&m_images[index]);
         painter.setRenderHint(QPainter::Antialiasing);
-        painter.setWindow(QRect(partRect.topLeft(), partRect.size() * qPainterEffectiveDevicePixelRatio(&painter)));
+        painter.setWindow(QRect(partRect.topLeft(), partRect.size() * effectiveDevicePixelRatio()));
         painter.setClipRect(rect);
         painter.save();
         // clear existing part
@@ -466,7 +466,7 @@ void SceneQPainterDecorationRenderer::resizeImages()
     client()->client()->layoutDecorationRects(left, top, right, bottom);
 
     auto checkAndCreate = [this](int index, const QSize &size) {
-        auto dpr = devicePixelRatio();
+        auto dpr = effectiveDevicePixelRatio();
         if (m_images[index].size() != size * dpr ||
             m_images[index].devicePixelRatio() != dpr)
         {

src/utils/common.cpp

@@ -181,11 +181,6 @@ Qt::KeyboardModifiers x11ToQtKeyboardModifiers(int state)
     return ret;
 }
 
-qreal qPainterEffectiveDevicePixelRatio(const QPainter *painter)
-{
-    return std::max(qreal(1), painter->device()->devicePixelRatioF());
-}
-
 QPoint popupOffset(const QRect &anchorRect, const Qt::Edges anchorEdge, const Qt::Edges gravity, const QSize popupSize)
 {
     QPoint anchorPoint;

src/utils/common.h

@@ -123,15 +123,6 @@ void KWIN_EXPORT ungrabXServer();
 bool KWIN_EXPORT grabXKeyboard(xcb_window_t w = XCB_WINDOW_NONE);
 void KWIN_EXPORT ungrabXKeyboard();
 
-/**
- * QPainter::setWindow() doesn't work as expected when the device pixel ratio of the paint
- * device is less than 1.
- *
- * QPainter simply doesn't allow the effective scale factor to be less than 1. Use this function
- * to determine the effective device pixel ratio by which the window rect has to be scaled.
- */
-qreal KWIN_EXPORT qPainterEffectiveDevicePixelRatio(const QPainter *painter);
-
 static inline QRegion mapRegion(const QMatrix4x4 &matrix, const QRegion &region)
 {
     QRegion result;