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This works around an AppKit bug in which mouseDown/mouseUp events are not correctly forwarded up the responder chain for views nested inside an NSPopover if (and only if) the macOS "Reduce Transparency" accessibility setting is enabled in the System Settings. When the above conditions are satisfied, the nested NSView receives the mouseDown:/mouseUp: call but if it delegates to the default implementation (implemented in NSResponder) mouseDown/mouseUp calls are triggered on containing views (in our case FlutterViewWrapper) but not triggered on the view controller and other responders in the responder chain until we an _NSPopoverWindow class is hit. A minimal AppKit-only (non-Flutter) repro shows this behaviour repros with even a minimal NSViewController implementation and an unmodified NSView. See: https://github.com/cbracken/PopoverRepro A radar has been filed with Apple and a copy posted to OpenRadar. See: http://www.openradar.me/FB12050037 In order to work around this bug, we override mouseDown/mouseUp in the topmost containing view of FlutterView (in our case, FlutterViewWrapper) to have the behaviour documented as the default behaviour in NSResponder's mouseDown/mouseUp documentation. In otherwords, to simply forward the call to self.nextResponder. See: https://developer.apple.com/documentation/appkit/nsresponder/1524634-mousedown Because replicating the exact configuration of a FlutterView contained in an NSPopover and System Settings that have been modified to enable the "Reduce Transparency" setting is difficult and likely error-prone in infra, we instead simulate the bug by testing that even if NSResponder's mouseDown/mouseUp method are swizzled to no-op, these calls are correctly forwarded to the next responder in the chain. If, in the future Apple does fix this issue, this workaround can be removed once Flutter's minimum supported macOS SDK is at least the version that contains the fix. Issue: https://github.com/flutter/flutter/issues/115015