Index: atoms.h
===================================================================
--- atoms.h
+++ atoms.h
@@ -88,6 +88,7 @@
     Xcb::Atom delete_atom;
     Xcb::Atom incr;
     Xcb::Atom wl_selection;
+    Xcb::Atom xwayland_randr_emu_monitor_rects;
 
     /**
      * @internal
Index: atoms.cpp
===================================================================
--- atoms.cpp
+++ atoms.cpp
@@ -81,6 +81,7 @@
     , wl_selection(QByteArrayLiteral("WL_SELECTION"))
     , m_dtSmWindowInfo(QByteArrayLiteral("_DT_SM_WINDOW_INFO"))
     , m_motifSupport(QByteArrayLiteral("_MOTIF_WM_INFO"))
+    , xwayland_randr_emu_monitor_rects(QByteArrayLiteral("_XWAYLAND_RANDR_EMU_MONITOR_RECTS"))
     , m_helpersRetrieved(false)
 {
 }
Index: geometry.cpp
===================================================================
--- geometry.cpp
+++ geometry.cpp
@@ -27,6 +27,7 @@
 
 */
 
+#include "atoms.h"
 #include "x11client.h"
 #include "composite.h"
 #include "cursor.h"
@@ -1576,8 +1577,49 @@
     c.window = window();
     c.x = m_clientGeometry.x();
     c.y = m_clientGeometry.y();
+
+    auto getEmulatedXWaylandSize = [this]() {
+        auto property = Xcb::Property(false, window(),
+                                      atoms->xwayland_randr_emu_monitor_rects, XCB_ATOM_CARDINAL,
+                                      0, 1000);
+        if (!property) {
+            return QSize();
+        }
+        uint32_t *rects = property.value<uint32_t*>();
+
+        if (property->value_len % 4) {
+            return QSize();
+        }
+
+        for (uint32_t i = 0; i < property->value_len / 4; i++) {
+            uint32_t *r = &rects[i];
+            if (r[0] - m_clientGeometry.x() == 0 && r[1] - m_clientGeometry.y() == 0) {
+                return QSize(r[2], r[3]);
+            }
+        }
+        return QSize();
+    };
+
     c.width = m_clientGeometry.width();
     c.height = m_clientGeometry.height();
+
+    if (isFullScreen()) {
+        // Workaround for XWayland clients setting fullscreen
+        const QSize emulatedSize = getEmulatedXWaylandSize();
+        if (emulatedSize.isValid()) {
+            c.width = emulatedSize.width();
+            c.height = emulatedSize.height();
+            const uint32_t values[] = { c.width, c.height };
+            ScopedCPointer<xcb_generic_error_t> error(xcb_request_check(connection(),
+                xcb_configure_window_checked(connection(), c.window,
+                                             XCB_CONFIG_WINDOW_WIDTH | XCB_CONFIG_WINDOW_HEIGHT,
+                                             values)));
+            if (!error.isNull()) {
+                qCDebug(KWIN_CORE) << "Error on emulating XWayland size: " << error->error_code;
+            }
+        }
+    }
+
     c.border_width = 0;
     c.above_sibling = XCB_WINDOW_NONE;
     c.override_redirect = 0;
Index: scene.cpp
===================================================================
--- scene.cpp
+++ scene.cpp
@@ -991,14 +991,21 @@
             if (rect.isValid()) {
                 sourceRect = QRectF(rect.topLeft() * textureScale,
                                     rect.bottomRight() * textureScale);
-            } else if (auto *buffer = surface->buffer()) {
-                // Try to get the source rectangle from the buffer size, what defines the source
-                // size without respect to destination size.
-                const auto origin = contentsRect.topLeft();
-                const QRectF rect = QRectF(origin, buffer->size() - QSize(origin.x(), origin.y()));
-                // Make sure a buffer was set already.
-                if (rect.isValid()) {
-                    sourceRect = rect;
+            } else {
+                auto *buffer = surface->buffer();
+                // XWayland client's geometry must be taken from their content placement since the
+                // buffer size is not in sync.
+                if (buffer && !toplevel->isClient()) {
+                    // Try to get the source rectangle from the buffer size, what defines the source
+                    // size without respect to destination size.
+                    const auto origin = contentsRect.topLeft();
+                    const QRectF rect = QRectF(origin,
+                                               buffer->size() - QSize(origin.x(), origin.y()));
+                    Q_ASSERT(rect.isValid());
+                    // Make sure a buffer was set already.
+                    if (rect.isValid()) {
+                        sourceRect = rect;
+                    }
                 }
             }
         }
Index: toplevel.h
===================================================================
--- toplevel.h
+++ toplevel.h
@@ -713,6 +713,7 @@
     bool m_isDamaged;
 
 private:
+    void handleXwaylandSurfaceSizeChange();
     // when adding new data members, check also copyToDeleted()
     QUuid m_internalId;
     Xcb::Window m_client;
Index: toplevel.cpp
===================================================================
--- toplevel.cpp
+++ toplevel.cpp
@@ -713,11 +713,13 @@
     using namespace KWayland::Server;
     if (m_surface) {
         disconnect(m_surface, &SurfaceInterface::damaged, this, &Toplevel::addDamage);
-        disconnect(m_surface, &SurfaceInterface::sizeChanged, this, &Toplevel::discardWindowPixmap);
+        disconnect(m_surface, &SurfaceInterface::sizeChanged,
+                   this, &Toplevel::handleXwaylandSurfaceSizeChange);
     }
     m_surface = surface;
     connect(m_surface, &SurfaceInterface::damaged, this, &Toplevel::addDamage);
-    connect(m_surface, &SurfaceInterface::sizeChanged, this, &Toplevel::discardWindowPixmap);
+    connect(m_surface, &SurfaceInterface::sizeChanged,
+            this, &Toplevel::handleXwaylandSurfaceSizeChange);
     connect(m_surface, &SurfaceInterface::subSurfaceTreeChanged, this,
         [this] {
             // TODO improve to only update actual visual area
@@ -735,6 +737,12 @@
     emit surfaceChanged();
 }
 
+void Toplevel::handleXwaylandSurfaceSizeChange()
+{
+    discardWindowPixmap();
+    Q_EMIT geometryShapeChanged(this, frameGeometry());
+}
+
 void Toplevel::addDamage(const QRegion &damage)
 {
     m_isDamaged = true;