Client: Avoid unnecessarily redrawing decorations

Note: QWaylandAbstractDecoration::update() doesn't actually update anything, it
just sets a dirty flag, and this is done explicitly in the other cases when the
decorations need to be redrawn as well (title and icon changes).

Fixes: QTBUG-75377
Change-Id: I2e8bd3abd3664c741e69a3e83ebf409872ddf31a
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 8 insertions, 5 deletions

diff --git a/src/client/qwaylandshmbackingstore.cpp b/src/client/qwaylandshmbackingstore.cpp
index 34044ec9b..c16d346eb 100644
--- a/src/client/qwaylandshmbackingstore.cpp
+++ b/src/client/qwaylandshmbackingstore.cpp
@@ -288,12 +288,15 @@ void QWaylandShmBackingStore::resize(const QSize &size)
         buffer = getBuffer(sizeWithMargins);
     }
 
-    qsizetype oldSize = mBackBuffer ? mBackBuffer->image()->sizeInBytes() : 0;
+    qsizetype oldSizeInBytes = mBackBuffer ? mBackBuffer->image()->sizeInBytes() : 0;
+    qsizetype newSizeInBytes = buffer->image()->sizeInBytes();
+
     // mBackBuffer may have been deleted here but if so it means its size was different so we wouldn't copy it anyway
-    if (mBackBuffer != buffer && oldSize == buffer->image()->sizeInBytes()) {
-        memcpy(buffer->image()->bits(), mBackBuffer->image()->constBits(), buffer->image()->sizeInBytes());
-    }
+    if (mBackBuffer != buffer && oldSizeInBytes == newSizeInBytes)
+        memcpy(buffer->image()->bits(), mBackBuffer->image()->constBits(), newSizeInBytes);
+
     mBackBuffer = buffer;
+
     // ensure the new buffer is at the beginning of the list so next time getBuffer() will pick
     // it if possible
     if (mBuffers.first() != buffer) {
@@ -301,7 +304,7 @@ void QWaylandShmBackingStore::resize(const QSize &size)
         mBuffers.prepend(buffer);
     }
 
-    if (windowDecoration() && window()->isVisible())
+    if (windowDecoration() && window()->isVisible() && oldSizeInBytes != newSizeInBytes)
         windowDecoration()->update();
 }