23.0.60; broken mouse-face highlighting in Gnus

[Stephen Berman]

1. emacs -Q
2. Eval this:
   (setq gnus-summary-line-format "%U%R%z%((%4L) %-20,20f \x2502 %*%B%s%)\n"
          gnus-sum-thread-tree-root "\x25b6 "
          gnus-sum-thread-tree-false-root "\x25b7 "
          gnus-sum-thread-tree-vertical "\x2502 "
          gnus-sum-thread-tree-leaf-with-other "\x251c\x2500\x25b8 ..."
          gnus-sum-thread-tree-single-leaf "\x2570\x2500\x25b8 ...")
3. M-x gnus, browse a group that has threaded messages and see the fancy
threading display produced by the above settings in the Summary buffer.
4. Select a subject header in the Summary buffer, such that the subject
contains one of the fancy threading characters, or one of the strings
"Re: ", "RE: ", "23.", "2.", and place the cursor on one of these
characters (on the first character in the strings "Re: ", "RE: ", "23.",
"2.").
5. Move the mouse over the subject header and observe the highlighting
everywhere except the character after the cursor (in the case of
"\x251c\x2500\x25b8 ..." and "\x2570\x2500\x25b8 ...", if the cursor is
over "\x2500", the both the immediately preceding and immediately
following characters are not highlighted).

Repeat the above in Emacs 23.0.50 but with the following settings in
step 2 (since the above are invalid characters in pre-unicode-2 Emacs):
   (setq gnus-summary-line-format "%U%R%z%((%4L) %-20,20f \x49022 %*%B%s%)\n"
            gnus-sum-thread-tree-root "\x490f6 "
            gnus-sum-thread-tree-false-root "\x490f7 "
            gnus-sum-thread-tree-leaf-with-other "\x4903c\x49020\x490fa ..."
            gnus-sum-thread-tree-vertical "\x49022"
            gnus-sum-thread-tree-single-leaf "\x490b0\x49020\x490fa ...")
Now at step 5 the entire subject shows mouse-face highlighting as it
should, i.e. no gaps.

Steve Berman