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

[Lukasz Kostyra]

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.

I would especially like to ask for someone macOS-and-ObjC-proficient to take a look at this - ObjC is not my strongest suite and I'm not 100% sure whether I did not introduce any memory leaks or other issues.

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PR Comment: https://git.openjdk.org/jfx/pull/1089#issuecomment-1505608558