fix: keep playlist reordering active while dragging

Sources/App/PlaybackQueue.swift

@@ -60,3 +60,47 @@ struct PlaybackQueue {
         }
     }
 }
+
+// Geometry stays in the unshifted content coordinate space throughout a gesture.
+struct PlaybackQueueDrag {
+    let source: Int
+    let frames: [CGRect]
+    let initialScrollOffset: CGFloat
+    private(set) var scrollOffset: CGFloat
+    private(set) var translation: CGFloat = 0
+    private(set) var destination: Int
+    var pointerY: CGFloat = 0
+
+    init(source: Int, frames: [CGRect], scrollOffset: CGFloat) {
+        self.source = source
+        self.frames = frames
+        initialScrollOffset = scrollOffset
+        self.scrollOffset = scrollOffset
+        destination = source
+    }
+
+    var offset: CGFloat { translation + scrollOffset - initialScrollOffset }
+
+    func center(at index: Int) -> CGFloat {
+        if index < source { return frames[index].minY + frames[source].height / 2 }
+        if index > source { return frames[index].maxY - frames[source].height / 2 }
+        return frames[source].midY
+    }
+
+    mutating func update(translation: CGFloat, scrollOffset: CGFloat) {
+        self.translation = translation
+        self.scrollOffset = scrollOffset
+        let centerY = frames[source].midY + offset
+        destination = frames.indices.min { abs(center(at: $0) - centerY) < abs(center(at: $1) - centerY) }!
+    }
+
+    var insertionOffset: Int { destination > source ? destination + 1 : destination }
+
+    func offset(at index: Int) -> CGFloat {
+        if index == source { return offset }
+        let target = destination
+        if source < index && index <= target { return -frames[source].height }
+        if target <= index && index < source { return frames[source].height }
+        return 0
+    }
+}