Prevent NSPopover from stealing an NSTextField’s focus when using the popover to display live search results. Swift adaption of Wil Shipley’s solution (https://stackoverflow.com/a/21111462)

CustomDocumentWindow.swift

protocol PopoverFirstResponderStealingSuppression {
    var suppressFirstResponderWhenPopoverShows: Bool { get }
}

class CustomDocumentWindow: NSWindow {
    override func makeFirstResponder(_ responder: NSResponder?) -> Bool {
        if responder != self.firstResponder,
            let newFirstResponder = responder as? NSView {
            let newFirstResponderWindow = newFirstResponder.window

            let currentFirstResponder = self.firstResponder
            var currentFirstResponderWindow: NSWindow

            if let currentFirstResponder = currentFirstResponder as? NSWindow {
                currentFirstResponderWindow = currentFirstResponder
            } else if let currentFirstResponder = currentFirstResponder as? NSView,
                let window = currentFirstResponder.window {
                currentFirstResponderWindow = window
            } else {
                return super.makeFirstResponder(newFirstResponder)
            }

            if newFirstResponderWindow != self
               && newFirstResponderWindow != currentFirstResponderWindow
               && self.currentEvent?.window != newFirstResponderWindow {

                var responderView: NSResponder? = newFirstResponder
                while responderView != nil {
                    if let responderView = responderView as? PopoverFirstResponderStealingSuppression,
                        responderView.suppressFirstResponderWhenPopoverShows {
                        return false
                    }
                    responderView = (responderView as? NSView)?.superview
                }
            }
        }

        return super.makeFirstResponder(responder)
    }
}

The NSWindow containing the NSTextField (or NSSearchField) must be subclassed as CustomDocumentWindow.

If you have an NSCollectionView inside the NSPopover, your NSCollectionView subclass must adapt the PopoverFirstResponderStealingSuppression protocol; not the NSPopover subclass.