diff --git a/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/misc/CloseBlocker.kt b/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/misc/CloseBlocker.kt
new file mode 100644
index 000000000..32c89c093
--- /dev/null
+++ b/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/misc/CloseBlocker.kt
@@ -0,0 +1,51 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2019-2020 JetBrains s.r.o.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+package org.jetbrains.projector.client.web.misc
+
+import org.w3c.dom.BeforeUnloadEvent
+import org.w3c.dom.Window
+import org.w3c.dom.events.Event
+
+class CloseBlocker(private val window: Window) {
+
+  private fun onBeforeUnload(e: Event) {
+    require(e is BeforeUnloadEvent)
+
+    e.preventDefault()
+    e.returnValue = ""
+  }
+
+  fun setListener() {
+    window.addEventListener(beforeUnloadType, this::onBeforeUnload)
+  }
+
+  fun removeListener() {
+    window.removeEventListener(beforeUnloadType, this::onBeforeUnload)
+  }
+
+  companion object {
+
+    private const val beforeUnloadType = "beforeunload"
+  }
+}
diff --git a/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/state/ClientState.kt b/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/state/ClientState.kt
index fab378d55..a9f2248dc 100644
--- a/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/state/ClientState.kt
+++ b/projector-client-web/src/main/kotlin/org/jetbrains/projector/client/web/state/ClientState.kt
@@ -57,6 +57,7 @@ import org.khronos.webgl.ArrayBuffer
 import org.w3c.dom.*
 import org.w3c.dom.events.Event
 import kotlin.browser.document
+import kotlin.browser.window
 import kotlin.math.roundToInt
 
 sealed class ClientState {
@@ -335,6 +336,10 @@ sealed class ClientState {
       else -> MobileKeyboardHelperImpl(openingTimeStamp, inputController.specialKeysState) { stateMachine.fire(ClientAction.AddEvent(it)) }
     }
 
+    private val closeBlocker = CloseBlocker(window).apply {
+      setListener()
+    }
+
     init {
       windowSizeController.addListener()
     }
@@ -458,6 +463,8 @@ sealed class ClientState {
 
         mobileKeyboardHelper.dispose()
 
+        closeBlocker.removeListener()
+
         showDisconnectedMessage(action.url, action.closeCode)
 
         Disconnected