RFR: 8233955: VM crashes if more than one file are added to ClipboardContent via drag and drop

[Kevin Rushforth]

On Wed, 12 Apr 2023 16:50:38 GMT, Lukasz Kostyra <lkostyra at openjdk.org> wrote:

> Crashes started happening due to macOS DnD API change from macOS 10.14 onwards. 10.14 incrodues some [DnD constrains](https://developer.apple.com/documentation/macos-release-notes/appkit-release-notes-for-macos-10_14#Drag-and-Drop) which our DnD code happened to trigger on some occasions.
> 
> Our code used deprecated `dragImage` API which since 10.14 had new requirement - each NSPasteboard item had to have a corresponding drag image. Not meeting this constraint raised an exception, which crashed the app. Since there was no possibility to add more drag images to `dragImage` API (it only took one NSImage as parameter) the code had to be rewritten - as such I upgraded it to new DnD API utilizing NSDraggingSession and related protocols.
> 
> One side-effect of new DnD API is that it now modifies NSPasteboard for us - previous behavior was more "separated" from user code perspective (write items to pasteboard -> initiate a drag via `dragImage`). Manually updating NSPasteboard before calling `beginDraggingSessionWithItems` raised another exception related to NSPasteboard already having DnD-ed elements inside it. Some system tests, however, relied on that behavior and writing to NSPasteboard (`MacPasteboardShim.java` used in some tests creates a `Clipboard.DND` for test purposes). Since this path is in tests I assumed this behavior should stay and tried to make it as close to working as possible. Tests (including those using `MacPasteboardShim`) pass after my changes.
> 
> Additionally, added a new manual test based on `DndTest.java` test which creates two temporary files and allows for testing faulty behavior.

This fixes the problem, but there is one regression in behavior I noticed. To reproduce:

1. Run `DndTest` (in the same manual tests dir as the one you added)
2. Hold down the CMD key
3. Drag the "DRAG ME" label to the "DROP ME" (while holding down CMD)

BUG: When you release the button, the drag does not complete. The expected behavior is that the drag completes with a transfer mode of "MOVE"

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PR Review: https://git.openjdk.org/jfx/pull/1089#pullrequestreview-1421453641