Dragging any e-mail or selection of e-mails to any QuickFolder fails silently.
I've tried repairing the affected folders and disabling and then re-enabling
QuickFolders but all to no avail.

The problem occurs only when I have the 'Phoenity Schredder' theme enabled -
not with the default theme and not with the 'TT DeepDark' theme.

Windows 8.1. QuickFolders 4.10 (but the problem beset the previous version of
QuickFolders too). Thunderbird 52.8

I reproduced it - seeems like all drop handlers are ignored. I found a warning
in Thunderbird's nsDragAndDrop.js but couldn't get the debugger to stop in it:

Tue Jun 26 2018 18:36:58
Error: TypeError: transferData.first is null
Source file: chrome://global/content/nsDragAndDrop.js
Line: 479

So something is dodgy in the way the script its loaded; my guess there is an
incorrect binding in one of the css modules of Phoenity.

Right. Thanks. I've emailed the Phoenity developer, directing him to this page.

The developer cannot reproduce the problem, and asks about my AddOns.
I've got these (in addition to QuickFolders)

Any Confirmation Message Before Send
Don't RestoreTabs
Enigmail
Folder Account
IMAP Draft Unread
Keep In Taskbar
LCD Clock
Mark Junk Read
MarkDown Here
Phoenity Buttons

And some others, but they are disabled.

Does this overlap with anything you have, Axel?

Also, I recently tried out, and then removed (replying-with-quote didn't work)
the latest Thunderbird beta. Perhaps that has something to do with the problem.

(In reply to comment #3)
> The developer cannot reproduce the problem, and asks about my AddOns.
> I've got these (in addition to QuickFolders)
> 
> And some others, but they are disabled.
> 
> Does this overlap with anything you have, Axel?
> 
No, but having the phoenix Theme seemed enough to trigger the error

> Also, I recently tried out, and then removed (replying-with-quote didn't work)
> the latest Thunderbird beta. Perhaps that has something to do with the problem.
> 
I don't think so... there is some problem with the Dragdrop handler in
thunderbird all I believe is that it somehow is connected to bindings as
phoenity has no JS code but does some bindings on toolbar buttons.
Unfortunately I don't know too much about them except that they are bad news
when they are removed, e.g. this recent change affected some of my Add-ons:

https://wiki.mozilla.org/Thunderbird/Add-ons_Guide_57

Changes in thunderbird59

The bindings for the toolbar customization have moved from "global" to
"messenger". If your add-on has customizable icons, please modify your
chrome.manifest as shown below:

style chrome://global/content/customizeToolbar.xul    chrome://.../skin/...
.css appversion<59.0a1
style chrome://messenger/content/customizeToolbar.xul chrome://.../skin/...
.css appversion>=59.0a1

There is a lot more that was moved between TB52 and TB60. It is possible that I
broke something in TB52 while fixing it for TB60 but if I did I should be able
to reproduce the error you are reporting.

I can't reproduce it, not on Windows, not on Linux, not when only QuickFolders
and Phoenity Shredder are enabled.

My guess still is it is some other extension playing up, can you test it with
only Phoenity Shredder and QuickFolders enabled?

(In reply to comment #5)
> There is a lot more that was moved between TB52 and TB60. It is possible that I
> broke something in TB52 while fixing it for TB60 but if I did I should be able
> to reproduce the error you are reporting.
> 
> I can't reproduce it, not on Windows, not on Linux, not when only QuickFolders
> and Phoenity Shredder are enabled.
> 
> My guess still is it is some other extension playing up, can you test it with
> only Phoenity Shredder and QuickFolders enabled?

I tried it - disabled all other (42) Add-ons, but it still happens.
(Thunderbird 52.8.0)