Bug 202418 – MouseEvent altKey property only changes on dragStart

• Component: UI Events
• Status: NEW

[Jay C]

Created attachment 379924 [details]
Screen recording of problem

Safari Version 13.0.1
macOS Version 10.14.6 (18G95)

While dragging a draggable element, `event.altKey` should change to true/false depending if I hold the option key on my Mac.

In Firefox, I can start a drag, and then toggle the option key to see the altKey value change from false to true. I do not need to have the option key pressed before I start dragging.

In Safari, I have to start the drag operation with option already held down in order for the MouseEvent's altKey property to be true. If I let go of the option key, the altKey value never changes. 

In Safari, if I start a drag event without holding the option key down, the MouseEvent's altKey property will never change. 

Here is a video logging out the value of altKey as I dragEnter and dragOver the items on the left. 

The item I am dragging has the following in its dragStart handler

```
e.dataTransfer.effectAllowed = 'copyMove'
e.dataTransfer.dropEffect = 'move'
```

the item I am dropping on has the following events. e.altKey will *always* be false in webkit if I don't start the drag operation with option held down. It will *always* be true if I start the drag operation with option held down.

```
function dropHandler(e) {
  e.preventDefault()
  e.dataTransfer.effectAllowed = 'copyMove'
  ...
  // If we're copying
  if (e.altKey) {
    fieldCopyHandler(oldDocumentId, thisDocumentId, fieldId)
  } else {
    fieldMoveHandler(oldDocumentId, thisDocumentId, fieldId)
  }
}

function dragEnterHandler(e) {
  console.log(e.altKey)
}

function dragOverHandler(e) {
  console.log(e.altKey)
}
```

I have attached a video

[Jay C]

I wrote out a long bug report when submitting this, but not seeing that text here so I will rewrite it (ugh).

Problem:
The MouseEvent's "altKey" property doesn't change when pressing/releasing the option key.

Example:

I have a set of list items that I can drag and drop, and another set of list items that can handle a drop.

in the drag start handler for the items I can drag, I have set the dataTransfer effectAllowed value to "copyMove".

in the drop Handler, dragEnter handler, and dragOver handler on the droppable list items, I am logging out the altKey property. 

If I start the drag operation with the option key held down, the altKey property will always be true, no matter if I release the key or not.

If I start the drag operation without the option key held down, the altKey property will always be false, no matter what I do.

I have attached a screen recording showing Firefox vs Safari

[Radar WebKit Bug Importer]

<rdar://problem/55909813>

[Jay C]

This may be related to https://bugs.webkit.org/show_bug.cgi?id=101853 as I am seeing that as well with the same dropevent

[Stefano Delli Ponti]

I confirm this bug also in Safari 13.1 and macOS 10.15.4.

The problem has been present in Safari for at least three years, perhaps more.
Unfortunately I don't remember the first version where it appeared, but I  am sure it was not present at the beginning. 

Both Chrome and Firefox allow to discover the values of the modifier keys after starting dragging, keeping them updated if changed later.

I think that a workaround can be to query the dataTransfer.effectAllowed, which is changed dynamically when toggling ctrl, alt or meta.

Unfortunately I didn't find a way to query for the shiftKey status.