Good idea. There's a few minor niggles, though:
1. Don't use `unsafeWindow`, this will break in Chrome and will not even work in Firefox+Greasemonkey if the effective `@grant` changes.
1. Avoid leading `*` wildcards in `@includes`, if you can help it. This is a big performance hit. `@match` limits the wildcard scope and performs much better and reports the sites/security correctly on Chrome. This gets all current SE sites:
// @match *://*.askubuntu.com/questions/*
// @match *://*.onstartups.com/questions/*
// @match *://*.serverfault.com/questions/*
// @match *://*.stackapps.com/questions/*
// @match *://*.stackexchange.com/questions/*
// @match *://*.stackoverflow.com/questions/*
// @match *://*.superuser.com/questions/*
1. The regex `match` needs to be more robust, to account for all legal characters in the URL, and for anchors.
1. Use injection so that the script will work on most userscript-capable browsers (Chrome, Firefox, Opera, Safari, etc.).
<br>
Here's a variant of the script I've been using. It also adds a "revisions" link since a post's history is sometimes hidden. It works on Chrome and Firefox, and should work on most other userscript-capable browsers:
###View [the source at GitHub Gist][1].
# [<kbd>Install from GitHub Gist</kbd>][2]
<br>
Change log:
<pre>
Versn Date Change
----- ---------- --------------------------------------------------------------------------
1.5 2019-08-28 Updated Metadata and Lint config for Tampermonkey changes and performance.
1.4 2019-08-28 SE renamed `short-link` to `js-share-link`.
1.3 2016-08-19 SE changed Timelines. Answers were separated out from the question timeline.
1.2 2015-07-22 Updated match list.
1.1 2014-02-07 Added revisions link. (On private copy, didn't update Gist until 2015.)
1.0 2013-06-11 Initial release to the public.
</pre>
[1]: https://gist.github.com/BrockA/5761948
[2]: https://gist.github.com/BrockA/5761948/raw/Add_Timeline_Button_to_Stack_Exchange_questions.user.js