Some kinds of display specifications specify something to display instead of the text that has the property. These are called replacing display specifications. Emacs does not allow the user to interactively move point into the middle of buffer text that is replaced in this way.
If a list of display specifications includes more than one replacing display specification, the first overrides the rest. Replacing display specifications make most other display specifications irrelevant, since those don’t apply to the replacement.
For replacing display specifications, the text that has the
property means all the consecutive characters that have the same
Lisp object as their display property; these characters are
replaced as a single unit. If two characters have different Lisp
objects as their display properties (i.e., objects which are
not eq), they are handled separately.
Here is an example which illustrates this point. A string serves as a replacing display specification, which replaces the text that has the property with the specified string (see Other Display Specifications). Consider the following function:
(defun foo ()
(dotimes (i 5)
(let ((string (concat "A"))
(start (+ i i (point-min))))
(put-text-property start (1+ start) 'display string)
(put-text-property start (+ 2 start) 'display string))))
This function gives each of the first ten characters in the buffer a
display property which is a string "A", but they don’t
all get the same string object. The first two characters get the same
string object, so they are replaced with one ‘A’; the fact that
the display property was assigned in two separate calls to
put-text-property is irrelevant. Similarly, the next two
characters get a second string (concat creates a new string
object), so they are replaced with one ‘A’; and so on. Thus, the
ten characters appear as five A’s.