Requested feature overview description.
On the upload page, make the upload preview float.
https://imgur.com/YAhBmsM
Why would it be useful?
At this time, the preview simply falls off the top of the page, and you may have to repeatedly scroll up and down between your tag suggestions and the image in order to tag it correctly as you skim through your suggestions.
What part(s) of the site page(s) are affected?
https://e621.net/post/upload

The same feature would be useful for editing posts.

Welp, suggested the exact same thing (+tagging) just two days prior.
https://e621.net/forum/show/251498

I'm going to help you now.

@-moz-document domain(e621.net) {

.tag-column:first-child {
height: auto !important; max-width: 45em !important;
column-count: 4 !important; -moz-column-count: 4 !important; -webkit-column-count: 4 !important;
column-gap: 10px !important; -moz-column-gap: 10px !important; -webkit-column-gap: 10px !important;
}

#upload_preview_img {position: fixed !important; width:45% !important;}

}

That is custom CSS. You can add it to your browser's CSS file or download an extension that lets you add a custom style sheet.

https://ffeathers.wordpress.com/2013/03/10/how-to-override-css-stylesheets-in-firefox/

The first part adds columns to the long-ass list of tag suggestions to make it easier to read with less scrolling.

The second part adds the floating preview image.

This CSS could be improved, but it should work immediately.

Picked around in chromes styles inspector and tested a handful of extensions but the preview is still static. Does seem to have done something though since the first of two lists of tags stretch down past the bottom border.

Looked through my chrome files, the css folder was no help unless I need to make a new css file. Also tried out relevant files in appdata which also yielded no results.

Won't hassle you with this so don't put too much energy into it. Just thought this would be something worth implementing for everyone.