Index: trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompTextField.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompTextField.java	(revision 18332)
+++ trunk/src/org/openstreetmap/josm/gui/tagging/ac/AutoCompTextField.java	(revision 18334)
@@ -158,4 +158,8 @@
             // do not autocomplete on control or deleted chars
             return;
+        if (getInputMethodRequests().getCommittedTextLength() != getDocument().getLength()) {
+            // do not autocomplete if there is uncommitted text (breaks Microsoft Japanese IME, see #21507)
+            return;
+        }
         if (!AUTOCOMPLETE_NUMBERS && IS_NUMBER.matcher(newText).matches())
             return;