x11window: round border size to integral XNative units

Both frameSize and clientSize are rounded to integral XNative units.
So their difference must also be rounded to integral XNative units.
Otherwise we get cycles of rounding that can cause growing window sizes.

BUG: 481460

src/utils/xcbutils.cpp

@@ -644,6 +644,11 @@ QSizeF fromXNative(const QSize &s)
     return QSizeF(fromXNative(s.width()), fromXNative(s.height()));
 }
 
+qreal nativeRound(qreal value)
+{
+    return fromXNative(toXNative(value));
+}
+
 static qreal nativeFloor(qreal value)
 {
     return std::floor(value * kwinApp()->xwaylandScale()) / kwinApp()->xwaylandScale();

src/utils/xcbutils.h

@@ -46,6 +46,13 @@ qreal KWIN_EXPORT fromXNative(int value);
 QRectF KWIN_EXPORT fromXNative(const QRect &value);
 QSizeF KWIN_EXPORT fromXNative(const QSize &value);
 
+/**
+ * Rounds a given value using the scale as a base
+ *
+ * Equivalent to fromXNative(toXNative(value))
+ */
+qreal KWIN_EXPORT nativeRound(qreal value);
+
 /** Floors a given value to using the scale as a base
  *  Use when flooring to ints from Xwayland
  *  i.e floor(a/scale) * scale

src/x11window.cpp

@@ -2807,8 +2807,8 @@ QPointF X11Window::framePosToClientPos(const QPointF &point) const
     qreal y = point.y();
 
     if (isDecorated()) {
-        x += borderLeft();
-        y += borderTop();
+        x += Xcb::nativeRound(borderLeft());
+        y += Xcb::nativeRound(borderTop());
     } else {
         x -= m_clientFrameExtents.left();
         y -= m_clientFrameExtents.top();
@@ -2823,8 +2823,8 @@ QPointF X11Window::clientPosToFramePos(const QPointF &point) const
     qreal y = point.y();
 
     if (isDecorated()) {
-        x -= borderLeft();
-        y -= borderTop();
+        x -= Xcb::nativeRound(borderLeft());
+        y -= Xcb::nativeRound(borderTop());
     } else {
         x += m_clientFrameExtents.left();
         y += m_clientFrameExtents.top();
@@ -2839,8 +2839,11 @@ QSizeF X11Window::frameSizeToClientSize(const QSizeF &size) const
     qreal height = size.height();
 
     if (isDecorated()) {
-        width -= borderLeft() + borderRight();
-        height -= borderTop() + borderBottom();
+        // Both frameSize and clientSize are rounded to integral XNative units
+        // So their difference must also be rounded to integral XNative units
+        // Otherwise we get cycles of rounding that can cause growing window sizes
+        width -= Xcb::nativeRound(borderLeft()) + Xcb::nativeRound(borderRight());
+        height -= Xcb::nativeRound(borderTop()) + Xcb::nativeRound(borderBottom());
     } else {
         width += m_clientFrameExtents.left() + m_clientFrameExtents.right();
         height += m_clientFrameExtents.top() + m_clientFrameExtents.bottom();
@@ -2855,8 +2858,11 @@ QSizeF X11Window::clientSizeToFrameSize(const QSizeF &size) const
     qreal height = size.height();
 
     if (isDecorated()) {
-        width += borderLeft() + borderRight();
-        height += borderTop() + borderBottom();
+        // Both frameSize and clientSize are rounded to integral XNative units
+        // So their difference must also be rounded to integral XNative units
+        // Otherwise we get cycles of rounding that can cause growing window sizes
+        width += Xcb::nativeRound(borderLeft()) + Xcb::nativeRound(borderRight());
+        height += Xcb::nativeRound(borderTop()) + Xcb::nativeRound(borderBottom());
     } else {
         width -= m_clientFrameExtents.left() + m_clientFrameExtents.right();
         height -= m_clientFrameExtents.top() + m_clientFrameExtents.bottom();