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Fix window cleanup logic on macOS (#164)
Instead of trying to detect when to clean up the window based on the `NSView`'s retain count, require window cleanup to be initiated explicitly via `Window::close`, `WindowHandle::close`, or `[NSWindowDelegate windowShouldClose:]` (in non-parented mode; called when the user clicks the "X" button). This fixes the leaks and use-after-frees that can be caused by the inherent unreliability of the retain count logic. As discussed in #153, this change essentially means that the `NSView` created by Baseview will not be suitable as the top-level view for an Audio Unit, since the Baseview API now requires that child windows be cleaned up by an explicit call to `WindowHandle::close`, and the only reliable signal for cleaning up an Audio Unit view is a call to `[NSView dealloc]`. However, this does not mean that Baseview cannot be used in the context of an Audio Unit; it just means that plugin frameworks must implement a compatibility layer with a wrapper `NSView` (which is the approach taken by JUCE). In order to implement this change: - `WindowState` is stored in an `Rc` rather than a `Box`. - `WindowHandle` holds an `Rc<WindowState>` so that `WindowHandle::close` can directly invoke window cleanup logic. - Since the window can be closed during an event handler, `WindowState::from_view` now returns a clone of the `Rc<WindowState>` held by the `NSView` to ensure that it lives until the end of an event handler. - In the non-parented case, the `NSView` is set as the window delegate, which allows it to receive the `windowShouldClose:` call when the user clicks the "X" button, upon which it will dispatch the `WillClose` event and initiate window cleanup logic. - `Window::open_parented` and `open_blocking` no longer `release` the `NSView` immediately after attaching it. Instead, the `NSView` is released as part of the cleanup logic in `WindowInner::close`. - `Window::resize` now checks if the window is open to avoid using the `NSView` after releasing it. - The overridden `release` method, the `retain_count_after_build` field, the `ParentHandle` struct, and the `close_requested` flag have all been removed.
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