bug#20260: 24.4.1; Triple-clicking selects and deselects the line in weird situations

[Jorgen Schaefer]

Hello!
We came across a weird behavior in Emacs.

The graphical version (in X) will select a line if the mouse is
triple-clicked. Under certain situations, it will select but immediately
deselect the line again.

Reproduction:

Triple-click on any line in any buffer. Notice that Emacs selects the
whole line, and leaves it selected.

Evaluate the following code.

(defun repro-filter (fun beg end delete)
  (let ((string (funcall fun beg end delete)))
    (with-temp-buffer
      (insert "."))
    string))

(with-current-buffer (get-buffer-create "*Bug*")
  (set (make-local-variable 'filter-buffer-substring-functions)
       '(repro-filter))
  (insert "Hello, World!\n")
  (pop-to-buffer (current-buffer)))

Triple-click on the "World" in the *Bug* buffer. Notice that Emacs
selects the whole line only momentarily, and then deselects it again
immediately.

I expected the line to stay selected like without this piece of code.

This bug was not present in 24.3.

Regards,
Jorgen