diff --git a/src/xpra/server/source/idle_mixin.py b/src/xpra/server/source/idle_mixin.py
index 15772713e7..611d89df7f 100644
--- a/src/xpra/server/source/idle_mixin.py
+++ b/src/xpra/server/source/idle_mixin.py
@@ -16,6 +16,12 @@
 
 class IdleMixin(StubSourceMixin):
 
+    @classmethod
+    def is_needed(cls, caps : typedict) -> bool:
+        #the 'keyboard' and 'mouse' capability were only added in v4,
+        #so we have to enable the mixin by default:
+        return caps.boolget("keyboard", True) or caps.boolget("mouse", True) or caps.boolget("windows", False)
+
     def __init__(self):
         self.idle_timeout = 0