Client: Fix buffer damage

If the specified damage rectangle has fractional coordinates in the buffer
local coordinate space, the buffer damage needs to be expanded, i.e.

- bufferRect.left = floor(rect.left * scale)
- bufferRect.right = ceil(rect.right * scale) = ceil((rect.x + rect.width) * scale)

Flooring the coordinates and ceiling the size is not enough. It can
produce incorrect results.

For example, consider that a rectangle with logical coordinates of
QRect(0, 23, 179, 46) has been damaged in a window with scale 1.5. When
flooring the coordinates and ceiling the size, the following buffer
damage rect will be produced: QRect(0, 34, 269, 69). Its height is off by
1, the expected height is 70 (ceil((23 + 46) * 1.5) - floor(23 * 1.5) =
ceil(103.5) - floor(34.5) = 104 - 34 = 70).

Change-Id: I927e75a2224bb58b4634125011d1305dbdfbb3aa
Reviewed-by: David Edmundson <davidedmundson@kde.org>
(cherry picked from commit d79db699866b37bd3e3358ca18a210dfc5c0b4b9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/client/qwaylandwindow.cpp b/src/client/qwaylandwindow.cpp
index cee87411b..f7ae77390 100644
--- a/src/client/qwaylandwindow.cpp
+++ b/src/client/qwaylandwindow.cpp
@@ -658,10 +658,15 @@ void QWaylandWindow::damage(const QRect &rect)
         return;
 
     const qreal s = scale();
-    if (mSurface->version() >= 4)
-        mSurface->damage_buffer(qFloor(s * rect.x()), qFloor(s * rect.y()), qCeil(s * rect.width()), qCeil(s * rect.height()));
-    else
+    if (mSurface->version() >= 4) {
+        const QRect bufferRect =
+                QRectF(s * rect.x(), s * rect.y(), s * rect.width(), s * rect.height())
+                        .toAlignedRect();
+        mSurface->damage_buffer(bufferRect.x(), bufferRect.y(), bufferRect.width(),
+                                bufferRect.height());
+    } else {
         mSurface->damage(rect.x(), rect.y(), rect.width(), rect.height());
+    }
 }
 
 void QWaylandWindow::safeCommit(QWaylandBuffer *buffer, const QRegion &damage)
@@ -699,8 +704,13 @@ void QWaylandWindow::commit(QWaylandBuffer *buffer, const QRegion &damage)
     attachOffset(buffer);
     if (mSurface->version() >= 4) {
         const qreal s = scale();
-        for (const QRect &rect: damage)
-            mSurface->damage_buffer(qFloor(s * rect.x()), qFloor(s * rect.y()), qCeil(s * rect.width()), qCeil(s * rect.height()));
+        for (const QRect &rect : damage) {
+            const QRect bufferRect =
+                    QRectF(s * rect.x(), s * rect.y(), s * rect.width(), s * rect.height())
+                            .toAlignedRect();
+            mSurface->damage_buffer(bufferRect.x(), bufferRect.y(), bufferRect.width(),
+                                    bufferRect.height());
+        }
     } else {
         for (const QRect &rect: damage)
             mSurface->damage(rect.x(), rect.y(), rect.width(), rect.height());