Created attachment 278102 [details]
We stumbled across this problem while working on an extension. We found out that if you start a drag from a web page, the draggesture and dragover event handlers are called correctly, but any chrome changes done in these handlers are not applied until the drag is canceled, or the drag occurs over the chrome. We know the handlers are being called because we logged the calls to disk with a timestamp.
Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US; rv:126.96.36.199) Gecko/20070725 Firefox/188.8.131.52
I'm attaching a testcase extension. To test simply install the extension and begin a drag. The extension should display and update the current pointer position in the status bar. I tested this on Windows and Linux, and it worked as expected. On Mac OS, the position is only updated while dragging over the chrome.
I can no longer reproduce this bug.