Fix RDP/VNC clipboard sync randomly stopping

Local copies were de-duplicated against the last synced text with no
expiry, so one lost send made re-copying the same text a no-op; every
open tab listened to the system clipboard, letting background sessions
overwrite it and forwarding one tab's update to all others; and RDP
never sent the initial Format List MS-RDPECLIP requires after Monitor
Ready, which the server can re-send mid-session (e.g. rdpclip.exe
restart), leaving sync dead until reconnect. Announce failures are now
logged to the event log instead of silently dropped.

Bump version to v2026.9.26.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/rdp_session_backend.cpp

@@ -2000,6 +2000,10 @@ void RdpSessionBackend::sendClipboardTextToRemote(rdp_freerdp* instance, const Q
     }
 
     if (cliprdrContext == nullptr || cliprdrContext->ClientFormatList == nullptr) {
+        // Kept in m_pendingLocalClipboardText and announced once the channel
+        // reports MonitorReady (see onCliprdrMonitorReady()).
+        emit eventLogged(QStringLiteral(
+            "RDP clipboard: channel not ready; copied text will be sent once it is."));
         return;
     }
 
@@ -2012,7 +2016,11 @@ void RdpSessionBackend::sendClipboardTextToRemote(rdp_freerdp* instance, const Q
     formatList.formats = &format;
 
     const UINT rc = cliprdrContext->ClientFormatList(cliprdrContext, &formatList);
-    Q_UNUSED(rc);
+    if (rc != CHANNEL_RC_OK) {
+        emit eventLogged(
+            QStringLiteral("RDP clipboard: failed to announce copied text to the server (0x%1).")
+                .arg(rc, 0, 16));
+    }
 #endif
 }
 
@@ -2324,6 +2332,29 @@ void RdpSessionBackend::onCliprdrMonitorReady()
     }
 
     emit eventLogged(QStringLiteral("RDP clipboard channel ready."));
+
+    QString pendingText;
+    {
+        std::lock_guard<std::mutex> guard(m_cliprdrMutex);
+        pendingText = m_pendingLocalClipboardText;
+    }
+    if (!pendingText.isEmpty()) {
+        sendClipboardTextToRemote(nullptr, pendingText);
+    } else if (cliprdrContext->ClientFormatList != nullptr) {
+        // MS-RDPECLIP requires the client to answer Monitor Ready with its
+        // capabilities *and* an initial Format List (empty is fine); the
+        // server doesn't treat the channel as initialized until it has
+        // both. The server can send Monitor Ready again mid-session -- e.g.
+        // when rdpclip.exe restarts -- and without this the clipboard
+        // stayed dead until the session was reconnected.
+        CLIPRDR_FORMAT_LIST emptyList = {};
+        const UINT rc = cliprdrContext->ClientFormatList(cliprdrContext, &emptyList);
+        if (rc != CHANNEL_RC_OK) {
+            emit eventLogged(
+                QStringLiteral("RDP clipboard: initial format list failed (0x%1).")
+                    .arg(rc, 0, 16));
+        }
+    }
 #endif
 }
 

src/session_tab.cpp

@@ -778,17 +778,29 @@ void SessionTab::onBackendUsernameRequested(const QString& prompt)
 
 void SessionTab::onBackendRemoteClipboardTextChanged(const QString& text)
 {
-    if (text == m_lastSyncedClipboardText) {
+    // Only the tab in front may touch the system clipboard. Every open
+    // RDP/VNC tab listens to it, and a background session re-announcing its
+    // own (stale) clipboard used to silently overwrite what was just copied
+    // locally, and an update applied by one tab was forwarded on to every
+    // other connected tab's remote as well.
+    if (!isVisible() || text.isEmpty()) {
         return;
     }
 
-    m_lastSyncedClipboardText = text;
-    QApplication::clipboard()->setText(text);
+    QClipboard* clipboard = QApplication::clipboard();
+    if (clipboard->text() == text) {
+        return;
+    }
+
+    m_pendingClipboardEcho = text;
+    m_pendingClipboardEchoMs = QDateTime::currentMSecsSinceEpoch();
+    clipboard->setText(text);
+    appendEvent(QStringLiteral("Clipboard: received %1 characters from remote.").arg(text.size()));
 }
 
 void SessionTab::onSystemClipboardChanged()
 {
-    if (!m_clipboardSyncSupported || m_state != SessionState::Connected) {
+    if (!m_clipboardSyncSupported || m_state != SessionState::Connected || !isVisible()) {
         return;
     }
 
@@ -798,12 +810,24 @@ void SessionTab::onSystemClipboardChanged()
     }
 
     const QString text = clipboard->text();
-    if (text == m_lastSyncedClipboardText) {
+    if (text.isEmpty()) {
+        return;
+    }
+
+    // Suppress only the change we caused ourselves, and only briefly. The
+    // old "same as last synced text" check had no expiry: once a send was
+    // lost (channel not ready, remote clipboard changed to something we
+    // never saw) copying that same text again did nothing, indefinitely.
+    constexpr qint64 kEchoWindowMs = 1000;
+    const bool isEcho = !m_pendingClipboardEcho.isEmpty() && text == m_pendingClipboardEcho
+        && QDateTime::currentMSecsSinceEpoch() - m_pendingClipboardEchoMs < kEchoWindowMs;
+    m_pendingClipboardEcho.clear();
+    if (isEcho) {
         return;
     }
 
-    m_lastSyncedClipboardText = text;
     emit requestSetClipboardText(text);
+    appendEvent(QStringLiteral("Clipboard: sending %1 characters to remote.").arg(text.size()));
 }
 
 void SessionTab::setupUi()

src/session_tab.h

@@ -119,7 +119,11 @@ private:
     SessionConnectOptions m_lastConnectOptions;
     QString m_terminalThemeName;
     int m_terminalFontPointSize;
-    QString m_lastSyncedClipboardText;
+    // Text we just wrote to the system clipboard from a remote update, so
+    // the dataChanged() it triggers isn't echoed straight back out. Only
+    // honored briefly -- see onSystemClipboardChanged().
+    QString m_pendingClipboardEcho;
+    qint64 m_pendingClipboardEchoMs = 0;
     bool m_clipboardSyncSupported;
 
     KodoTerm* m_sshTerminal;