1649134 - DragSession can end while paused in debugger

Status: UNCONFIRMED

(DevTools :: Debugger, defect, P3)

Description

[Rob Hoelz]

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:77.0) Gecko/20100101 Firefox/77.0

Steps to reproduce:

• Go to https://mdn.github.io/dom-examples/drag-and-drop/copy-move-DataTransfer.html
• Open the debugger in devtools, browse to copy-move-DataTransfer.html
• Set a breakpoint on line 48 (should be console.log("Drop") in function drop_handler)
• Try to drag the "select this element and drag to the Copy Drop Zone" into the Copy Drop Zone
• The breakpoint is reached - if you evaluate ev.dataTransfer in the console, you get null, and if you continue execution of the script, you get an error:

TypeError: ev.dataTransfer is null

Actual results:

ev.dataTransfer is null

Expected results:

ev.dataTransfer should be a valid DataTransfer object

Comment 1

[Rob Hoelz]

This also happens if I make a copy of that page and insert a debugger; line at line 48.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

Comment 3

[Logan Smyth [:loganfsmyth]]

What is happening is that the drag processing logic does not pause properly, so while the drop handler is paused at line 48, the dragend handler will fire, calling ev.dataTransfer.clearData(); on line 68, so that when you resume and get to line 52, the dataTransfer data has been erased already.