diff --git a/abstract_client.cpp b/abstract_client.cpp
index 0ba96360e6..f86dd84389 100644
--- a/abstract_client.cpp
+++ b/abstract_client.cpp
@@ -1147,7 +1147,7 @@ QSize AbstractClient::sizeForClientSize(const QSize &wsize, Sizemode mode, bool
 {
     Q_UNUSED(mode)
     Q_UNUSED(noframe)
-    return wsize;
+    return wsize + QSize(borderLeft() + borderRight(), borderTop() + borderBottom());
 }
 
 void AbstractClient::addRepaintDuringGeometryUpdates()
diff --git a/autotests/integration/shell_client_test.cpp b/autotests/integration/shell_client_test.cpp
index ee56f67aca..db480c3b7f 100644
--- a/autotests/integration/shell_client_test.cpp
+++ b/autotests/integration/shell_client_test.cpp
@@ -346,6 +346,7 @@ void TestShellClient::testFullscreen()
     QVERIFY(!c->isFullScreen());
     QCOMPARE(c->clientSize(), QSize(100, 50));
     QCOMPARE(c->isDecorated(), decoMode == ServerSideDecoration::Mode::Server);
+    QCOMPARE(c->sizeForClientSize(c->clientSize()), c->geometry().size());
     QSignalSpy fullscreenChangedSpy(c, &ShellClient::fullScreenChanged);
     QVERIFY(fullscreenChangedSpy.isValid());
     QSignalSpy geometryChangedSpy(c, &ShellClient::geometryChanged);