Disclosure: written with AI assistance (Claude) to fix one machine (Fedora 45, Plasma 6.8.2, krfb 26.08.3, one monitor 2560x1440 at 125%). Tested on that setup only. Not a recommended fix; posted in case it helps. Context: KDE bugs 466276 (comment 13) and 524406.
It may need small manual changes on other krfb versions (e.g. 26.04.x).

--- a/events/xdp/xdpevents.cpp
+++ b/events/xdp/xdpevents.cpp
@@ -15,6 +15,10 @@
 
 #include <QApplication>
 #include <QGlobalStatic>
+#include <QGuiApplication>
+#include <QScreen>
+#include <QSettings>
+#include <QStandardPaths>
 
 class EventData
 {
@@ -45,6 +49,40 @@
                                                                                      QStringLiteral("/org/freedesktop/portal/desktop"), QDBusConnection::sessionBus()));
 }
 
+// The VNC client reports pointer positions in framebuffer (physical) pixels, but the
+// RemoteDesktop portal expects the compositor's logical (scaled) coordinates.
+// Divide by the display scale. An optional override can be set in ~/.config/krfbrc:
+//   [General]
+//   pointerScale=1.25
+// Without it, the scale is derived from the captured frame width and the logical screen width.
+static qreal configuredPointerScale()
+{
+    static const qreal value = []() -> qreal {
+        QSettings cfg(QStandardPaths::writableLocation(QStandardPaths::GenericConfigLocation) + QStringLiteral("/krfbrc"), QSettings::IniFormat);
+        bool ok = false;
+        const qreal v = cfg.value(QStringLiteral("pointerScale")).toDouble(&ok);
+        return (ok && v > 0) ? v : 0;
+    }();
+    return value;
+}
+
+static qreal pointerScale(int frameBufferWidth)
+{
+    const qreal configured = configuredPointerScale();
+    if (configured > 0) {
+        return configured;
+    }
+    // QScreen::devicePixelRatio() is rounded up to a whole number on Wayland (2 for a 1.25x
+    // output), so derive the scale from the captured frame width and the logical screen width.
+    if (const QScreen *screen = QGuiApplication::primaryScreen()) {
+        const int logicalWidth = screen->geometry().width();
+        if (logicalWidth > 0 && frameBufferWidth > 0) {
+            return qreal(frameBufferWidth) / logicalWidth;
+        }
+    }
+    return 1.0;
+}
+
 void XdpEventHandler::handleKeyboard(bool down, rfbKeySym keySym)
 {
     const QDBusObjectPath sessionHandle = frameBuffer()->customProperty(QStringLiteral("session_handle")).value<QDBusObjectPath>();
@@ -61,7 +99,8 @@
     }
 
     if (x != data->x || y != data->y) {
-        data->dbusXdpRemoteDesktopService->NotifyPointerMotionAbsolute(sessionHandle, QVariantMap(), streamNodeId, x, y);
+        const qreal scale = pointerScale(frameBuffer()->width());
+        data->dbusXdpRemoteDesktopService->NotifyPointerMotionAbsolute(sessionHandle, QVariantMap(), streamNodeId, x / scale, y / scale);
         data->x = x;
         data->y = y;
     }