1420590 - dragleave event returns bad target

Status: RESOLVED FIXED

(Core :: DOM: Copy & Paste and Drag & Drop, defect, P2)

Description

[Danny Lin]

User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0
Build ID: 20171024165158

Steps to reproduce:

1. download the attachment and open it in Firefox.
2. drag something into the page and then drag out.


Actual results:

When dragleave fires, the event.target is set to document (if e10s activated) or XULDocument (if e10s not activated; and accessing event.target throws an error in this case).


Expected results:

The event.target should be the .dropmask element. (in other browsers)

Comment 1

[Danny Lin]

Attachment: Illustration of the dragleave bug

Comment 2

[Neil Deakin]

Within EventStateManager::FireDragEnterOrExit an event is created and AssignDragEventData is called to copy over data from the existing event. However, the event target in mTarget is sometimes not the same as the one passed to EventDispatcher::Dispatch. Specifically, for dragexit/dragleave events, it is sLastDragOverFrame's content.

It does seem to work if I manually null out mTarget and mOriginalTarget before calling EventDispatcher::Dispatch.

Is this a suitable fix? Should I just do this in all cases? That is, something like the following for dragexit/dragleave events:

  event.AssignDragEventData(*aDragEvent, true);
+ event.mTarget = nullptr;
+ event.mOriginalTarget = nullptr;

Comment 3

[Olli Pettay [:smaug][[e-mail]]]

Hmm, is this a regression from bug 1327185, sort of.

Comment 4

[Olli Pettay [:smaug][[e-mail]]]

But yes, that looks reasonable. Or one could pass false to AssignDragEventData and set only the needed *Target variables.

Comment 5

[Ming-Chou Shih [:stone]]

Attachment: bug-1420590-fix-v1.patch

Comment 6

[Ming-Chou Shih [:stone]]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=0bd5181f1cf91d12def8547a5fe76e3d48346417

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/9b1af0e28db1
dragleave event returns bad target. r=smaug.

Comment 8

[Dorel Luca [:dluca]]

https://hg.mozilla.org/mozilla-central/rev/9b1af0e28db1