* Expected behavior

In a terminal
$ echo pwet | xclip -i -selection clipboard
# This command fills the clipboard selection with "pwet"
Start conkeror, you can paste the clipboard selection with C-y, it works fine.

* The Bug
$ echo pwet | xclip -i -selection clipboard -l 1 -verbose

# This command fills the clipboard selection with "pwet" and waits until
# it is yanked. Once the clipboard selection has been read, it is cleared.

Start conkeror, if you try to paste the clipboard selection with C-y, it triggers xclip (xclip ends) but
the content isn't pasted.

The problem is that Conkeror reads the clipboard selection twice, this can be tested by using the
following command:
$ echo pwet | xclip -i -selection clipboard -l 2 -verbose
# The -l X option is used to control how many times the clipboard selection
# can be read before it is cleared and xclip ended.

When invoked like this, as expected, the content is correctly yanked with C-y.