+ (`common-part
+ ;; Can't just use 0 when base-size (see above) is non-zero.
+ (let ((start (if (get-text-property 0 'face arg)
+ 0
+ (next-single-property-change 0 'face arg))))
+ (when start
+ ;; completions-common-part comes first, but we can't just look for this
+ ;; value because it can be in a list.
+ (or
+ (let ((value (get-text-property start 'face arg)))
+ (text-property-not-all start (length arg)
+ 'face value arg))
+ (length arg)))))