diff --git a/autotests/integration/shell_client_test.cpp b/autotests/integration/shell_client_test.cpp
--- a/autotests/integration/shell_client_test.cpp
+++ b/autotests/integration/shell_client_test.cpp
@@ -109,6 +109,7 @@
     void testXdgInitialState();
     void testXdgInitiallyMaximised();
     void testXdgInitiallyMinimized();
+    void testXdgWindowGeometry();
 };
 
 void TestShellClient::initTestCase()
@@ -1463,6 +1464,41 @@
     QVERIFY(c->isMinimized());
 }
 
+void TestShellClient::testXdgWindowGeometry()
+{
+    QScopedPointer<Surface> surface(Test::createSurface());
+    QScopedPointer<XdgShellSurface> shellSurface(Test::createXdgShellStableSurface(surface.data(), nullptr, Test::CreationSetup::CreateOnly));
+    QSignalSpy configureRequestedSpy(shellSurface.data(), &XdgShellSurface::configureRequested);
+    surface->commit(Surface::CommitFlag::None);
+
+    configureRequestedSpy.wait();
+    shellSurface->ackConfigure(configureRequestedSpy.first()[2].toUInt());
+
+    // Create a 160x140 window in with a margin of 10(left), 20(top), 30(right), 40(bottom). Giving a total buffer size 200, 100
+    shellSurface->setWindowGeometry(QRect(10, 20, 160, 40));
+    auto c = Test::renderAndWaitForShown(surface.data(), QSize(200,100), Qt::blue);
+    configureRequestedSpy.wait(); //window activated after being shown
+
+    QSignalSpy geometryChangedSpy(c, &ShellClient::geometryChanged);
+    // resize to 300,200 in kwin terms
+    c->setGeometry(QRect(100, 100, 300, 200));
+    QVERIFY(configureRequestedSpy.wait());
+    // requested geometry should not include the margins we had above
+    const QSize requestedSize = configureRequestedSpy.last()[0].value<QSize>();
+    QCOMPARE(requestedSize, QSize(300, 200) - QSize(10 + 30, 20 + 40));
+    shellSurface->ackConfigure(configureRequestedSpy.last()[2].toUInt());
+    Test::render(surface.data(), requestedSize + QSize(10 + 30, 20 + 40), Qt::blue);
+    geometryChangedSpy.wait();
+
+    // kwin's concept of geometry should remain the same
+    QCOMPARE(c->geometry(), QRect(100, 100, 300, 200));
+
+    c->setFullScreen(true);
+    configureRequestedSpy.wait();
+    // when full screen, the window geometry (i.e without margins) should fill the screen
+    const QSize requestedFullScreenSize = configureRequestedSpy.last()[0].value<QSize>();
+    QCOMPARE(requestedFullScreenSize, QSize(1280, 1024));
+}
 
 WAYLANDTEST_MAIN(TestShellClient)
 #include "shell_client_test.moc"
diff --git a/shell_client.h b/shell_client.h
--- a/shell_client.h
+++ b/shell_client.h
@@ -214,14 +214,17 @@
     void setTransient();
     bool shouldExposeToWindowManagement();
     void updateClientOutputs();
+    void updateWindowMargins();
     KWayland::Server::XdgShellSurfaceInterface::States xdgSurfaceStates() const;
     void updateShowOnScreenEdge();
     void updateMaximizeMode(MaximizeMode maximizeMode);
     // called on surface commit and processes all m_pendingConfigureRequests up to m_lastAckedConfigureReqest
     void updatePendingGeometry();
     QPoint popupOffset(const QRect &anchorRect, const Qt::Edges anchorEdge, const Qt::Edges gravity, const QSize popupSize) const;
     static void deleteClient(ShellClient *c);
 
+    QSize toWindowGeometry(const QSize &geometry) const;
+
     KWayland::Server::ShellSurfaceInterface *m_shellSurface;
     KWayland::Server::XdgShellSurfaceInterface *m_xdgShellSurface;
     KWayland::Server::XdgShellPopupInterface *m_xdgShellPopup;
@@ -291,6 +294,8 @@
     QString m_captionSuffix;
     QHash<qint32, PingReason> m_pingSerials;
 
+    QMargins m_windowMargins;
+
     bool m_compositingSetup = false;
 };
 
diff --git a/shell_client.cpp b/shell_client.cpp
--- a/shell_client.cpp
+++ b/shell_client.cpp
@@ -372,6 +372,7 @@
     SurfaceInterface *s = surface();
     disconnect(s, &SurfaceInterface::committed, this, &ShellClient::finishInit);
 
+    updateWindowMargins();
     if (!isInitialPositionSet()) {
         QRect area = workspace()->clientArea(PlacementArea, Screens::self()->current(), desktop());
         placeIn(area);
@@ -427,6 +428,16 @@
     delete c;
 }
 
+QSize ShellClient::toWindowGeometry(const QSize &size) const
+{
+    QSize adjustedSize = size - QSize(borderLeft() + borderRight(), borderTop() + borderBottom());
+    // a client going fullscreen should have the window the contents size of the screen
+    if (!isFullScreen() && requestedMaximizeMode() != MaximizeFull) {
+        adjustedSize -= QSize(m_windowMargins.left() + m_windowMargins.right(), m_windowMargins.top() + m_windowMargins.bottom());
+    }
+    return adjustedSize;
+}
+
 QStringList ShellClient::activities() const
 {
     // TODO: implement
@@ -519,6 +530,7 @@
     auto s = surface();
     if (s->size().isValid()) {
         m_clientSize = s->size();
+        updateWindowMargins();
         updatePendingGeometry();
     }
     markAsMapped();
@@ -617,11 +629,11 @@
         // reset geometry to the one before blocking, so that we can compare properly
         geom = geometryBeforeUpdateBlocking();
     }
-    // TODO: better merge with Client's implementation
     const QSize requestedClientSize = QSize(w, h) - QSize(borderLeft() + borderRight(), borderTop() + borderBottom());
+    const QSize requestedWindowGeometrySize = toWindowGeometry(QSize(w, h));
 
     if (requestedClientSize == m_clientSize && !isWaitingForMoveResizeSync() &&
-        (m_requestedClientSize.isEmpty() || requestedClientSize == m_requestedClientSize)) {
+        (m_requestedClientSize.isEmpty() || requestedWindowGeometrySize == m_requestedClientSize)) {
         // size didn't change, and we don't need to explicitly request a new size
         doSetGeometry(QRect(x, y, w, h));
         updateMaximizeMode(m_requestedMaximizeMode);
@@ -1107,7 +1119,7 @@
 
     QSize size;
     if (rect.isValid()) {
-        size = rect.size() - QSize(borderLeft() + borderRight(), borderTop() + borderBottom());
+        size = toWindowGeometry(rect.size());
     } else {
         size = QSize(0, 0);
     }
@@ -1126,7 +1138,7 @@
         if (parent) {
             const QPoint globalClientContentPos = parent->geometry().topLeft() + parent->clientPos();
             const QPoint relativeOffset = rect.topLeft() - globalClientContentPos;
-            serialId = m_xdgShellPopup->configure(QRect(relativeOffset, rect.size()));
+            serialId = m_xdgShellPopup->configure(QRect(relativeOffset, size));
         }
     }
 
@@ -1867,6 +1879,34 @@
     surface()->setOutputs(clientOutputs);
 }
 
+void ShellClient::updateWindowMargins()
+{
+    QRect windowGeometry;
+    QSize clientSize = m_clientSize;
+
+    if (m_xdgShellSurface) {
+        windowGeometry = m_xdgShellSurface->windowGeometry();
+    } else if (m_xdgShellPopup) {
+        windowGeometry = m_xdgShellPopup->windowGeometry();
+        if (!clientSize.isValid()) {
+            clientSize = m_xdgShellPopup->initialSize();
+        }
+    } else {
+        return;
+    }
+
+    if (windowGeometry.isEmpty() ||
+        windowGeometry.width() > clientSize.width() ||
+        windowGeometry.height() > clientSize.height()) {
+        m_windowMargins = QMargins();
+    } else {
+        m_windowMargins = QMargins(windowGeometry.left(),
+                                    windowGeometry.top(),
+                                    clientSize.width() - (windowGeometry.right() + 1),
+                                    clientSize.height() - (windowGeometry.bottom() + 1));
+    }
+}
+
 bool ShellClient::isPopupWindow() const
 {
     if (Toplevel::isPopupWindow()) {