Steps to reproduce the issue

go to the extension installer.
inspect source and remove the class="hidden" from the div with id="loading"

Expected result

The joomla logo animated gif is displayed in the middle of the screen

Actual result

The gif is not displayed because of an error in the js which puts the loading div below the installer-install div instead of on top of it.

For the style top to be created this should be appended with + 'px'

But that offset makes no sense and as far as I can tell it should simply be top: 0

What is needed is an offsetleft which takes into account the width of the sidebar and then the loader will be displayed in the middle of the main div whether the sidebar is expanded or not.

Sadly my js knowledge is not good enough for that