Open Radar

 
Summary:
During a drag and drop session of an application being debugged while Xcode is in full-screen mode, if the app pauses in the debugger, you can’t get to Xcode to fix it. This is an OS X issue as much as an Xcode issue, but I don’t expect the circumstances around it happen very often with any other task.

It appears that the OS does not expect anything to pause or interrupt a dragging session, ever. This makes it extremely difficult to debug drag and drop in my apps using my preferred setup of Xcode in full-screen mode.

Steps to Reproduce:
1. Open an Xcode project that does something with drag and drop.
2. Ensure the window is in full screen mode.
3. Set a breakpoint that will trigger during the drag, e.g. in -draggingUpdated:; my favourite example of how dangerous this can be is to do something that throws an exception, and have an exception breakpoint.
4. Build and run the application.
5. Perform the drag and drop such that the breakpoint is triggered.

Expected Results:
I expected the system to switch me to Xcode’s space so that I could see the full-screen mode window for the app and interact with the app in the debugger.

Actual Results:
Instead, the system became basically unusable:
1. I cannot open Mission Control.
2. I cannot use gestures or configured keyboard shortcuts to move to Xcode’s window.
3. If Terminal is open in full-screen mode, or if it’s not currently open but window restoration is on and it was most recently open with a full-screen mode window, then it cannot (easily) be activated to allow you to kill the debug server (cf Notes, below).
4. I cannot ⌘⇥ to full-screen applications, including Xcode.
5. I cannot kill the app which is being debugged.
6. Even after working around this issue (again, cf Notes, below), Mission Control and full-screen apps behave strangely: For example, I cannot reliably activate full-screen apps with ⌘⇥ or by clicking Dock icons until I have selected them manually with Mission Control.

Regression:
This did not occur with 10.6, where there were no full-screen apps but there were Spaces.

Notes:
One workaround for the trigger is to not use Xcode in full-screen mode. This is my usual process, but it is not really a solution; it’s still possible to lock it up without realizing, humans and exceptions being what they are, and ultimately dropping out of full-screen mode for the sake of debugging this is untenable since it doesn’t allow you to properly debug interactions with full-screen apps. (I have not explicitly tested my app in full-screen mode during drag-and-drop, but I would expect similar behaviour.)

Once the actual lock-up has happened, the only workaround I have found so far is to use Terminal or DTerm (usually DTerm, since my Terminal is full-screen!) to run `killall debugserver`, which results in the app being killed and the system resuming most of its functionality.