import SwiftUI
import AppKit

// The within-window material glass Messages.app uses for its chat header and
// composer footer: an NSVisualEffectView blending within the window, frosting
// whatever scrolls behind it. Deliberately NO colour of its own — a tint can
// only ever mismatch the feed (it did: windowBackgroundColor isn't what the
// pane actually sits on). A blur of a solid is that solid, so the glass
// matches the transcript at rest and frosts what passes beneath.
//
// ⚠️ Two dead ends, so nobody re-walks them:
// - alphaValue < 1 on an NSVisualEffectView silently DISABLES its backdrop
//   blur — you get a faded plate with sharp content bleeding through, not a
//   lighter frost. The band must run at exactly 1.
// - A true CSS-style backdrop-filter (pure gaussian via public
//   CALayer.backgroundFilters, no material plate) is compositor-ignored in
//   this window tree; only the private CABackdropLayer gets backdrop
//   sampling, and we don't ship private API.
struct FrostedBand: View {
    var body: some View { WithinWindowBlur() }
}

private struct WithinWindowBlur: NSViewRepresentable {
    // Decoration must be transparent to the mouse. This AppKit view lives in
    // its own PlatformViewHost, and SwiftUI does not keep representable
    // NSViews' z-order in step with the SwiftUI stack — after a composer-row
    // re-mount the band's host could land ABOVE the field's scroll view in
    // NSView order, and every click on the input died on the glass (and
    // resigned the caret to the window): the "composer goes unclickable for
    // a while" bug. hitTest nil = clicks always fall through to whatever the
    // band is dressing, whatever the subview order.
    private final class ClickThroughEffectView: NSVisualEffectView {
        override func hitTest(_ point: NSPoint) -> NSView? { nil }
    }

    func makeNSView(context: Context) -> NSVisualEffectView {
        let v = ClickThroughEffectView()
        v.blendingMode = .withinWindow
        v.material = .headerView
        v.state = .active
        return v
    }
    func updateNSView(_ nsView: NSVisualEffectView, context: Context) {}
}