Here is a screenshot of how it should look:
https://commons.wikimedia.org/wiki/File:Mobile_sidebar_preview_-_Show_page_in_mobile_view_while_browsing_the_desktop_site_-_bigger_arrow.png

Cannot reproduce the problem on Firefox 44 on Fedora Linux:

You might want to disable other potentially interfering gadgets and check your browser's developer console for any error output. Also note that gadget issues are on-wiki and might not receive support in Phabricator.

Only works with Vector skin. At Pref-Appearance-Skins, I did ''Preview'' on all of them & fails on all except Vector. ~~~~

See the developer console of any web browser, after enabling a different skin than Vector (e.g. Modern):

TypeError: $content.position(...) is undefined

Hence the line

var top = $content.position().top,

in https://meta.wikimedia.org/wiki/User:Brion_VIBBER/mobile-sidebar.js (which is called from https://en.wikipedia.org/wiki/MediaWiki:Gadget-mobile-sidebar.js ) needs to be fixed by anybody interested in fixing this.

As that is a personal script by a user hosted on-wiki, this is unfortunately out of scope for Phabricator. You could contact the author or commented on the discussion page of the script.

Found here:
https://meta.wikimedia.org/wiki/User_talk:Brion_VIBBER#Mobile_Sidebar_not_showing_up_in_Modern_skin

Mobile Sidebar not showing up in Modern skin

Hi Brion and {{ping|Brion Vibber (WMF)}}.

{{tracked|T126553}}

We are having trouble displaying the Mobile Sidebar using the 'Modern' skin. Works fine in 'Vector'.

We want to roll it out as a [[:en:Wikipedia talk:TOTD|Wikipedia Tip-of-the-Day]] soon for folks to try out. Got any ideas? Ping me back on the English Wikipedia. Cheers! [[User:Checkingfax|Checkingfax]] ([[User talk:Checkingfax|talk]]) 21:04, 11 February 2016 (UTC)

:this may or may not be the source of this specific problem, but the script has this line:

<syntaxhighlight lang=javascript>var $content = $('#content');</syntaxhighlight>

as far as i understand, what you want to do there is

<syntaxhighlight lang=javascript>var $content = mw.util.$content;</syntaxhighlight>

peace - [[User:קיפודנחש|קיפודנחש]] ([[User talk:קיפודנחש|talk]]) 20:31, 15 February 2016 (UTC)

::on further reading: $content is not used for anything useful - its only use is to calculate top, which in turn is not used. so the real fix is to remove those two variables. peace - [[User:קיפודנחש|קיפודנחש]] ([[User talk:קיפודנחש|talk]]) 01:28, 16 February 2016 (UTC)

:::on further further reading, $content is indeed not used, but it should be, to replace all the

<code>$(#content)</code>

in the code... peace - [[User:קיפודנחש|קיפודנחש]] ([[User talk:קיפודנחש|talk]]) 01:54, 16 February 2016 (UTC)

I'm going to keep this on my short list but I've got too many things in play to commit to self-assigning it until I finish a few others. If anyone else can work on this before, feel free to take the bug and ping me if necessary for questions/advice!

@brion :
please read your user page on meta. i created a patched version of your script that works with all skins, although i could not fit the icon sanely on the non-vector ones, so i resorted to text.

in addition to the non-vector issue, there's also a small issue with RTL.
both are discussed in your meta talk page. i will be glad to throw my patched version away and use the original when it's not needed.

peace.