applied Andreas Amann's patch from Oct 2010, thanks

dwm.c

@@ -42,7 +42,7 @@
 
 /* macros */
 #define BUTTONMASK              (ButtonPressMask|ButtonReleaseMask)
-#define CLEANMASK(mask)         (mask & ~(numlockmask|LockMask))
+#define CLEANMASK(mask)         (mask & (ShiftMask|ControlMask|Mod1Mask|Mod2Mask|Mod3Mask|Mod4Mask|Mod5Mask))
 #define INRECT(X,Y,RX,RY,RW,RH) ((X) >= (RX) && (X) < (RX) + (RW) && (Y) >= (RY) && (Y) < (RY) + (RH))
 #define ISVISIBLE(C)            ((C->tags & C->mon->tagset[C->mon->seltags]))
 #define LENGTH(X)               (sizeof X / sizeof X[0])