User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

In Developer Edition, drag from the site identity area or drag a link from the page and hover the Bookmarks Menu Button.

Actual results:

The #BMB_bookmarksPopup doesn't auto-open. Reported on SUMO at https://support.mozilla.org/questions/1266993

Expected results:

The #BMB_bookmarksPopup should auto-open.

In case helpful, button:

<toolbarbutton id="bookmarks-menu-button" 
ondragenter="PlacesMenuDNDHandler.onDragEnter(event);" 
ondragover="PlacesMenuDNDHandler.onDragOver(event);" 
ondragleave="PlacesMenuDNDHandler.onDragLeave(event);" 
ondrop="PlacesMenuDNDHandler.onDrop(event);" 
oncommand="BookmarkingUI.onCommand(event);" 
removable="true" cui-areatype="toolbar">

mozregression result:

2019-08-18T17:48:59: INFO : application_vendor: Mozilla
2019-08-18T17:48:59: INFO : application_version: 69.0a1
2019-08-18T17:48:59: INFO : platform_buildid: 20190522232535
2019-08-18T17:48:59: INFO : platform_changeset: c8e9b6a81194dff2d37b4f67d23a419fd4587e49
2019-08-18T17:48:59: INFO : platform_repository: https://hg.mozilla.org/integration/mozilla-inbound
2019-08-18T17:48:59: INFO : platform_version: 69.0a1
2019-08-18T17:49:36: INFO : Narrowed inbound regression window from [f287bb6c, d4a9b4dd] (3 builds) to [f287bb6c, c8e9b6a8] (2 builds) (~1 steps left)
2019-08-18T17:49:36: DEBUG : Starting merge handling...
2019-08-18T17:49:36: DEBUG : Using url: https://hg.mozilla.org/integration/mozilla-inbound/json-pushes?changeset=c8e9b6a81194dff2d37b4f67d23a419fd4587e49&full=1
2019-08-18T17:49:37: DEBUG : Found commit message:
Bug 1519577 Convert toolbarbutton to a custom element r=surkov

Differential Revision: https://phabricator.services.mozilla.com/D31941

Potentially this is a similar issue/fix to folders on the Bookmarks Toolbar not opening on hover (bug 1555496). But I don't understand this code, so...

Andrew, Your bunch of patch seems to cause the regression. Can you please look into this?

Andrew is no longer with Mozilla. Alexander, can you maybe take a look at this as the reviewer of the regressing patch? Otherwise, feel free to redirect as necessary. Thanks!

(In reply to jscher2000 from comment #0)

Potentially this is a similar issue/fix to folders on the Bookmarks Toolbar not opening on hover (bug 1555496). But I don't understand this code, so...

Indeed it was similar issue. If you are comfortable with JS code, then little of debugging usually helps for issues like this one, for sure as long as you have entry point, which is PlacesMenuDNDHandler.onDragEnter in this case as you mentioned above.

https://hg.mozilla.org/mozilla-central/rev/32bc8be194ac

https://hg.mozilla.org/releases/mozilla-beta/rev/32bc8be194ac

Reproduced the initial issue on Firefox 69.0.1.
Verified fixed on Windows 7 x64, Ubuntu 18.04 x64 and macOS 10.14 using Firefox 70 Beta 9 (20190923154733).