Thanks for taking the time to report this!

Could you please summarize these on-wiki conversations by creating one task per problem, with steps to reproduce, expected outcome and actual outcome? Also see https://www.mediawiki.org/wiki/How_to_report_a_bug - Thanks!
For the steps to reproduce here, could you please also open your web browser's developer tools and reload the page on which you see the problem? If there is a problem or an error with JavaScript it should be printed in the 'console' of the developer tools. For more information please see:

• Firefox ≥24
• Internet Explorer
• Google Chrome
• Apple Safari
• Opera

Regarding the error posted at https://en.wikisource.org/wiki/Wikisource:Scriptorium/Help#Strange_happenings_when_using_.27Show_preview.27,

In windows 7 and Xubuntu 14.04 both 32bit, Firefox 48.01b and 48.02b changes the proofreading layout when clicked on 'Select preview'.

Chrome in Windows 7 and Chromium in Xubuntu, doesn't have this problem.

Wouldn't 1.28.0-wmf.7 have been deployed to this wiki on the 22nd, not the 21st, of June?

22nd of June correlates the day when the problems began. Just curious, what made you mention this date as opposed to the 21?

https://www.mediawiki.org/wiki/MediaWiki_1.28/Roadmap shows this would've been deployed to enwikisource Wednesday, 22 June 2016.

changes the proofreading layout when clicked on 'Select preview'

Changes to what? I don't see anything wrong. Could so send us screenshots? Could you also disable all your gadgets/user scripts and see if the error still happens.

Image of one of the changes: https://en.wikisource.org/wiki/File:Change_of_edit_layout.jpg

This bug report was changed and some of my comments and the participants were removed.

Thank you for the image. Do you have any error in your debug console? Instruction for Firefox: https://developer.mozilla.org/en-US/docs/Tools/Web_Console

In T138554#2406370, @Ineuw wrote:

This bug report was changed and some of my comments and the participants were removed.

This bug report in Wikimedia Phabricator has not seen any removals (and I do not know how that would be relevant).

Just to comment to AKlapper and Tpt: I don't know in which time zone/country you both reside, but I need some sleep on occasion. Before this bug report, I didn't know that you wanted the output of Firefox's web web console. This is the contents of the web console when I click to preview the edit changes THE FIRST TIME! on this page:https://en.wikisource.org/w/index.php?title=Page:Popular_Science_Monthly_Volume_48.djvu/869&action=edit

and I will not alter this page so that you can follow my steps.

Exception in module-execute in module mediawiki.action.edit:load.php:178:464
TypeError: $form.serializeObject is not a function TypeError: $form.serializeObject is not a function
Stack trace:
[object Object]load.php:178:502

SyntaxError: nothing to repeatindex.php:234:12

Just clicking on preview again without any additional editing, the web console output is:

SyntaxError: nothing to repeat index.php:234:12

If you need any additional info, just let me know which tab output of the web tools you want, and I can paste it here or in pastebin.

Unchecked all gadgets, cleared my common.js and common.css, but to no avail. The problem is the same. The text layout switches from horizontal on the bottom view, to side by side, and loosing the enhanced editing bar. I always use the vector skin, but also tried with the monobook skin but the problem is the same.

These are the short results copied from the console:

console js.txt

TypeError: $form.serializeObject is not a function
load.php:158:1334

console net.txt

Exception in module-execute in module mediawiki.action.edit:load.php:178:464
TypeError: $form.serializeObject is not a function TypeError: $form.serializeObject is not a function
Stack trace:

[object Object]load.php:178:502

I generated css console outputs twice for comparison and they are different. So posted them to pastebin as follows:

http://pastebin.com/2x8RBJuQ as console css 1.txt

http://pastebin.com/N5Mcw7Lu as console css 2.txt

I really don't know what else I can do.

Was this issue investigated? Is there any additional info missing, which would help to have this issue resolved?