x11window: Calculate the native floor from the begining of the output

If this function is useful to make sure we are properly aligned with the
output's pixels, we should start with the output's 0, otherwise we'll be
carrying over the rounding errors increasingly as our workspace grows.

BUG: 459373

src/core/output.cpp

@@ -154,6 +154,16 @@ QRect Output::mapFromGlobal(const QRect &rect) const
     return rect.translated(-geometry().topLeft());
 }
 
+QRectF Output::mapFromGlobal(const QRectF &rect) const
+{
+    return rect.translated(-geometry().topLeft());
+}
+
+QRectF Output::mapToGlobal(const QRectF &rect) const
+{
+    return rect.translated(geometry().topLeft());
+}
+
 Output::Capabilities Output::capabilities() const
 {
     return m_information.capabilities;

src/core/output.h

@@ -109,6 +109,16 @@ public:
      */
     QRect mapFromGlobal(const QRect &rect) const;
 
+    /**
+     * Maps the specified @a rect from the global coordinate system to the output-local coords.
+     */
+    QRectF mapFromGlobal(const QRectF &rect) const;
+
+    /**
+     * Maps a @a rect in this output coordinates to the global coordinate system.
+     */
+    QRectF mapToGlobal(const QRectF &rect) const;
+
     /**
      * Returns a short identifiable name of this output.
      */

src/utils/xcbutils.cpp

@@ -9,6 +9,9 @@
 */
 #include "utils/xcbutils.h"
 #include "utils/common.h"
+#include <core/output.h>
+#include <workspace.h>
+
 // Qt
 #include <QDebug>
 // xcb
@@ -641,15 +644,17 @@ QSizeF fromXNative(const QSize &s)
     return QSizeF(fromXNative(s.width()), fromXNative(s.height()));
 }
 
-qreal nativeFloor(qreal value)
+static qreal nativeFloor(qreal value)
 {
     return std::floor(value / kwinApp()->xwaylandScale()) * kwinApp()->xwaylandScale();
 }
 
-QRectF nativeFloor(const QRectF &value)
+QRectF nativeFloor(const QRectF &rect)
 {
-    return QRectF(nativeFloor(value.left()), nativeFloor(value.top()),
+    const auto output = workspace()->outputAt(rect.center());
-                  nativeFloor(value.width()), nativeFloor(value.height()));
+    const QRectF outputRect = output->mapFromGlobal(rect);
+    return output->mapToGlobal(QRectF(nativeFloor(outputRect.left()), nativeFloor(outputRect.top()),
+                                      nativeFloor(outputRect.width()), nativeFloor(outputRect.height())));
 }
 
 } // namespace Xcb

src/utils/xcbutils.h

@@ -43,7 +43,6 @@ QSizeF KWIN_EXPORT fromXNative(const QSize &value);
  *  Use when flooring to ints from Xwayland
  *  i.e floor(a/scale) * scale
  */
-qreal KWIN_EXPORT nativeFloor(qreal value);
 QRectF KWIN_EXPORT nativeFloor(const QRectF &value);
 
 // forward declaration of methods