diff --git a/app/ui.js b/app/ui.js
index 48d9af8e..e7072d19 100644
--- a/app/ui.js
+++ b/app/ui.js
@@ -274,6 +274,9 @@ const UI = {
             return mouseButtonMapper;
         }
 
+        mouseButtonMapper.set(0, XVNC_BUTTONS.LEFT_BUTTON);
+        mouseButtonMapper.set(1, XVNC_BUTTONS.MIDDLE_BUTTON);
+        mouseButtonMapper.set(2, XVNC_BUTTONS.RIGHT_BUTTON);
         mouseButtonMapper.set(3, XVNC_BUTTONS.BACK_BUTTON);
         mouseButtonMapper.set(4, XVNC_BUTTONS.FORWARD_BUTTON);
         WebUtil.writeSetting("mouseButtonMapper", mouseButtonMapper.dump());
diff --git a/core/mousebuttonmapper.js b/core/mousebuttonmapper.js
index 565e813b..b69475a4 100644
--- a/core/mousebuttonmapper.js
+++ b/core/mousebuttonmapper.js
@@ -1,15 +1,19 @@
 export const XVNC_BUTTONS = {
-    LEFT_BUTTON: 0,
-    MIDDLE_BUTTON: 1,
-    RIGHT_BUTTON: 2,
-    TURN_SCROLL_WHEEL_UP: 3,
-    TURN_SCROLL_WHEEL_DOWN: 4,
-    PUSH_SCROLL_WHEEL_LEFT: 5,
-    PUSH_SCROLL_WHEEL_RIGHT: 6,
-    BACK_BUTTON: 7,
-    FORWARD_BUTTON: 8
+    LEFT_BUTTON: 1,
+    MIDDLE_BUTTON: 2,
+    RIGHT_BUTTON: 3,
+    TURN_SCROLL_WHEEL_UP: 4,
+    TURN_SCROLL_WHEEL_DOWN: 5,
+    PUSH_SCROLL_WHEEL_LEFT: 6,
+    PUSH_SCROLL_WHEEL_RIGHT: 7,
+    BACK_BUTTON: 8,
+    FORWARD_BUTTON: 9
 };
 
+export function xvncButtonToMask(xvncButton) {
+    return 1 << (xvncButton - 1);
+}
+
 export default class MouseButtonMapper {
     constructor() {
         this.map = new Map();
diff --git a/core/rfb.js b/core/rfb.js
index 252beb04..90a95549 100644
--- a/core/rfb.js
+++ b/core/rfb.js
@@ -26,7 +26,7 @@ import DES from "./des.js";
 import KeyTable from "./input/keysym.js";
 import XtScancode from "./input/xtscancodes.js";
 import { encodings } from "./encodings.js";
-import MouseButtonMapper from "./mousebuttonmapper.js";
+import { MouseButtonMapper, xvncButtonToMask } from "./mousebuttonmapper.js";
 
 import RawDecoder from "./decoders/raw.js";
 import CopyRectDecoder from "./decoders/copyrect.js";
@@ -1578,11 +1578,11 @@ export default class RFB extends EventTargetMixin {
                 this.checkLocalClipboard();
 
                 this._handleMouseButton(pos.x, pos.y,
-                                        true, 1 << mappedButton);
+                                        true, xvncButtonToMask(mappedButton));
                 break;
             case 'mouseup':
                 this._handleMouseButton(pos.x, pos.y,
-                                        false, 1 << mappedButton);
+                                        false, xvncButtonToMask(mappedButton));
                 break;
             case 'mousemove':
                 this._handleMouseMove(pos.x, pos.y);