diff --git a/lcl/interfaces/gtk2/gtk2callback.inc b/lcl/interfaces/gtk2/gtk2callback.inc
index cf3a8a92df..60001581b3 100644
--- a/lcl/interfaces/gtk2/gtk2callback.inc
+++ b/lcl/interfaces/gtk2/gtk2callback.inc
@@ -2094,8 +2094,11 @@ var
       (csDesigning in AWinControl.ComponentState) then
         exit;
 
-    MessI.Msg := CheckMouseButtonDownUp({%H-}TLCLIntfHandle(widget), AWinControl, LastMouse, EventXY, MouseButton, True);
+    MessI.Msg := CheckMouseButtonDownUp({%H-}TLCLIntfHandle(widget), AWinControl,
+                                        LastMouse, EventXY, MouseButton, True);
     MessI.Keys := MessI.Keys or BtnKey;
+    if BtnKey in [MK_XBUTTON1, MK_XBUTTON2] then
+      MessI.Keys :=  MessI.Keys or BtnKey shl 11;
     case LastMouse.ClickCount of
       2: MessI.Keys := MessI.Keys or MK_DOUBLECLICK;
       3: MessI.Keys := MessI.Keys or MK_TRIPLECLICK;
@@ -2155,7 +2158,7 @@ begin
       2: if not CheckMouseButtonDown(3, MK_MBUTTON) then Exit;
       3: if not CheckMouseButtonDown(2, MK_RBUTTON) then Exit;
       8: if not CheckMouseButtonDown(4, MK_XBUTTON1) then Exit;
-      9: if not CheckMouseButtonDown(4, MK_XBUTTON2) then Exit;
+      9: if not CheckMouseButtonDown(5, MK_XBUTTON2) then Exit;
       else
         begin
           MessI.Msg := LM_NULL;
@@ -2300,7 +2303,7 @@ begin
     2: if not CheckMouseButtonUp(3, LM_MBUTTONUP) then Exit;
     3: if not CheckMouseButtonUp(2, LM_RBUTTONUP) then exit;
     8: if not CheckMouseButtonUp(4, LM_XBUTTONUP) then exit;
-    9: if not CheckMouseButtonUp(4, LM_XBUTTONUP) then exit;
+    9: if not CheckMouseButtonUp(5, LM_XBUTTONUP) then exit;
   else
     begin
       MessI.Msg := LM_NULL;