diff --git a/app/backend/computermanager.cpp b/app/backend/computermanager.cpp
index 7e82e0de9..187c6e6d5 100644
--- a/app/backend/computermanager.cpp
+++ b/app/backend/computermanager.cpp
@@ -397,6 +397,8 @@ void ComputerManager::startPolling()
     });
     connect(m_MultiSeatDiscovery, &MultiSeatDiscovery::aboutToPoll,
             this, &ComputerManager::updateMultiSeatProbeTargets);
+    // start() only queues the first poll. It must not poll synchronously: this function holds
+    // m_Lock for write, and updateMultiSeatProbeTargets() takes it for read.
     m_MultiSeatDiscovery->start();
 
     // Start polling threads for each known host
@@ -438,6 +440,9 @@ void ComputerManager::startPollingComputer(NvComputer* computer)
 // beside share one address — probing it twice would just double the requests for nothing. Local
 // addresses only: seat ports are not port-forwarded, so probing a remote address would be four
 // guaranteed failures per tick against someone else's network.
+//
+// ⛔ Takes m_Lock for read. It must never run on a thread that already holds m_Lock, because
+// QReadWriteLock is not recursive and that thread would wait on itself.
 void ComputerManager::updateMultiSeatProbeTargets()
 {
     if (m_MultiSeatDiscovery == nullptr) {
diff --git a/app/backend/multiseatdiscovery.cpp b/app/backend/multiseatdiscovery.cpp
index af1eb6c6e..a22046fe5 100644
--- a/app/backend/multiseatdiscovery.cpp
+++ b/app/backend/multiseatdiscovery.cpp
@@ -26,8 +26,15 @@ MultiSeatDiscovery::~MultiSeatDiscovery()
 void MultiSeatDiscovery::start()
 {
     m_Timer->start();
-    // Poll immediately on start, don't wait for first interval
-    poll();
+
+    // Poll soon after start rather than waiting a full interval, but from the event loop, never
+    // from inside start().
+    //
+    // ⛔ Do not call poll() directly here. ComputerManager::startPolling() calls start() while it
+    // holds m_Lock for write, and poll() emits aboutToPoll(), whose slot takes that same lock for
+    // read on the same thread. QReadWriteLock is not recursive, so the main thread waited on
+    // itself forever: 6.3.4 hung ("Not Responding") on every launch.
+    QMetaObject::invokeMethod(this, &MultiSeatDiscovery::poll, Qt::QueuedConnection);
 }
 
 void MultiSeatDiscovery::stop()
@@ -42,6 +49,11 @@ void MultiSeatDiscovery::setHostsToProbe(const QStringList& addresses)
 
 void MultiSeatDiscovery::poll()
 {
+    // The first poll is queued by start(). If stop() ran before it was delivered, do nothing.
+    if (!m_Timer->isActive()) {
+        return;
+    }
+
     // Refresh the address list first — hosts may have been added or removed since the last tick.
     emit aboutToPoll();