Fix v2.0 for #2256:

An alternative fix - last one broke stuff that I only discovered
*after* committing! Go figure!

Basically, gave B_REDR events for the sliders -- I still think
there is a bigger underlying issue here, but it is best left for
Ton the interface guru! ;)

source/blender/src/interface.c

@@ -3468,7 +3468,7 @@ static int ui_do_block(uiBlock *block, uiEvent *uevent)
 			
 						if( (block->flag & UI_BLOCK_LOOP) && but->type==BLOCK);
 						else	
-							retval= UI_RETURN_OK; /* event handled, so return */
+							if (butevent) retval= UI_RETURN_OK;
 					}
 				}
 			}