Add a hover badge and switcher for stacked windows

Rectangle.xcodeproj/project.pbxproj

@@ -92,6 +92,8 @@
 		9824704F22B189250037B409 /* BestEffortWindowMover.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9824704A22B189250037B409 /* BestEffortWindowMover.swift */; };
 		9824705122B28D7A0037B409 /* LeftRightHalfCalculation.swift in Sources */ = {isa = PBXBuildFile; fileRef = 9824705022B28D7A0037B409 /* LeftRightHalfCalculation.swift */; };
 		983BBD6F253B609D000D223E /* FootprintWindow.swift in Sources */ = {isa = PBXBuildFile; fileRef = 983BBD6E253B609D000D223E /* FootprintWindow.swift */; };
+		F1AC0DE000000000000000A2 /* StackBadgeGeometry.swift in Sources */ = {isa = PBXBuildFile; fileRef = F1AC0DE000000000000000A1 /* StackBadgeGeometry.swift */; };
+		F1AC0DE000000000000000A4 /* StackBadgeManager.swift in Sources */ = {isa = PBXBuildFile; fileRef = F1AC0DE000000000000000A3 /* StackBadgeManager.swift */; };
 		983DD04028A844BE00BF1EEE /* SnapAreaViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = 983DD03F28A844BE00BF1EEE /* SnapAreaViewController.swift */; };
 		983DD04428B0639E00BF1EEE /* SnapAreaModel.swift in Sources */ = {isa = PBXBuildFile; fileRef = 983DD04328B0639E00BF1EEE /* SnapAreaModel.swift */; };
 		984EDB0F29A42ED200D119D2 /* LaunchOnLogin.swift in Sources */ = {isa = PBXBuildFile; fileRef = 984EDB0E29A42ED200D119D2 /* LaunchOnLogin.swift */; };
@@ -317,6 +319,8 @@
 		9824704A22B189250037B409 /* BestEffortWindowMover.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BestEffortWindowMover.swift; sourceTree = "<group>"; };
 		9824705022B28D7A0037B409 /* LeftRightHalfCalculation.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LeftRightHalfCalculation.swift; sourceTree = "<group>"; };
 		983BBD6E253B609D000D223E /* FootprintWindow.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FootprintWindow.swift; sourceTree = "<group>"; };
+		F1AC0DE000000000000000A1 /* StackBadgeGeometry.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = StackBadgeGeometry.swift; sourceTree = "<group>"; };
+		F1AC0DE000000000000000A3 /* StackBadgeManager.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = StackBadgeManager.swift; sourceTree = "<group>"; };
 		983DD03F28A844BE00BF1EEE /* SnapAreaViewController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SnapAreaViewController.swift; sourceTree = "<group>"; };
 		983DD04328B0639E00BF1EEE /* SnapAreaModel.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SnapAreaModel.swift; sourceTree = "<group>"; };
 		984EDB0E29A42ED200D119D2 /* LaunchOnLogin.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LaunchOnLogin.swift; sourceTree = "<group>"; };
@@ -708,6 +712,7 @@
 				98910B3C2311304D0066EC23 /* PrefsWindow */,
 				989DA30C243FC0C3008C7AA4 /* WelcomeWindow */,
 				98C2755F231FF6AE009B9292 /* Snapping */,
+				F1AC0DE000000000000000A5 /* StackBadge */,
 				98D4B6C925B625B6009C7BF6 /* TodoMode */,
 				985B9BFA22BE218C00A2E8F0 /* Popover */,
 				982140EA22B7DB0400ABFB3F /* WindowMover */,
@@ -786,6 +791,15 @@
 			path = TodoMode;
 			sourceTree = "<group>";
 		};
+		F1AC0DE000000000000000A5 /* StackBadge */ = {
+			isa = PBXGroup;
+			children = (
+				F1AC0DE000000000000000A1 /* StackBadgeGeometry.swift */,
+				F1AC0DE000000000000000A3 /* StackBadgeManager.swift */,
+			);
+			path = StackBadge;
+			sourceTree = "<group>";
+		};
 /* End PBXGroup section */
 
 /* Begin PBXNativeTarget section */
@@ -1081,6 +1095,8 @@
 				9824702C22AFA22E0037B409 /* AccessibilityWindowController.swift in Sources */,
 				988D066122EB4C7C004EABD7 /* FirstThirdCalculation.swift in Sources */,
 				983BBD6F253B609D000D223E /* FootprintWindow.swift in Sources */,
+				F1AC0DE000000000000000A2 /* StackBadgeGeometry.swift in Sources */,
+				F1AC0DE000000000000000A4 /* StackBadgeManager.swift in Sources */,
 				988D067F22EB4EDE004EABD7 /* MoveLeftRightCalculation.swift in Sources */,
 				AA3A9E8B29230A82004EB8E5 /* CFExtension.swift in Sources */,
 				98C6DEF023CE191700CC0C1E /* GapCalculation.swift in Sources */,

Rectangle/AccessibilityElement.swift

@@ -250,6 +250,16 @@ class AccessibilityElement {
     var isMinimized: Bool? {
         windowElement?.wrappedElement.getValue(.minimized) as? Bool
     }
+
+    var title: String? {
+        wrappedElement.getValue(.title) as? String
+    }
+
+    /// Caps how long AX calls through this element can block on an
+    /// unresponsive app (the systemwide default is several seconds).
+    func setMessagingTimeout(_ seconds: Float) {
+        AXUIElementSetMessagingTimeout(wrappedElement, seconds)
+    }
 
     var isFullScreen: Bool? {
         guard let subrole = windowElement?.getElementValue(.fullScreenButton)?.subrole else { return nil }

Rectangle/AppDelegate.swift

@@ -25,6 +25,7 @@ class AppDelegate: NSObject, NSApplicationDelegate {
     private var applicationToggle: ApplicationToggle!
     private var windowCalculationFactory: WindowCalculationFactory!
     private var snappingManager: SnappingManager!
+    private var stackBadgeManager: StackBadgeManager!
     private var titleBarManager: TitleBarManager!
     private var greenButtonManager: GreenButtonManager!
 
@@ -153,6 +154,7 @@ class AppDelegate: NSObject, NSApplicationDelegate {
         self.shortcutManager = ShortcutManager(windowManager: windowManager)
         self.applicationToggle = ApplicationToggle(shortcutManager: shortcutManager)
         self.snappingManager = SnappingManager()
+        self.stackBadgeManager = StackBadgeManager()
         self.titleBarManager = TitleBarManager()
         self.greenButtonManager = GreenButtonManager()
         self.initializeTodo()

Rectangle/Defaults.swift

@@ -66,6 +66,7 @@ class Defaults {
     static let cyclingOverlapOffset = OptionalBoolDefault(key: "cyclingOverlapOffset")
     static let cyclingOverlapOffsetSize = FloatDefault(key: "cyclingOverlapOffsetSize", defaultValue: 11)
     static let cyclingOverlapMaxCascade = IntDefault(key: "cyclingOverlapMaxCascade", defaultValue: 1)
+    static let stackBadge = OptionalBoolDefault(key: "stackBadge")
     static let fullIgnoreBundleIds = JSONDefault<[String]>(key: "fullIgnoreBundleIds")
     static let notifiedOfProblemApps = BoolDefault(key: "notifiedOfProblemApps")
     static let specifiedHeight = FloatDefault(key: "specifiedHeight", defaultValue: 1050)
@@ -194,6 +195,7 @@ class Defaults {
         cyclingOverlapOffset,
         cyclingOverlapOffsetSize,
         cyclingOverlapMaxCascade,
+        stackBadge,
         moveFixedSizeToEdge,
         greenButtonOverride
     ]

Rectangle/PrefsWindow/SettingsViewController.swift

@@ -117,6 +117,11 @@ class SettingsViewController: NSViewController {
         Notification.Name.showAdditionalSizesInMenuChanged.post()
     }
 
+    @objc func toggleStackBadge(_ sender: NSButton) {
+        Defaults.stackBadge.enabled = sender.state == .on
+        Notification.Name.stackBadgeChanged.post()
+    }
+
     @objc func toggleCyclingOverlapOffset(_ sender: NSButton) {
         Defaults.cyclingOverlapOffset.enabled = sender.state == .on
     }
@@ -901,11 +906,17 @@ class SettingsViewController: NSViewController {
             overlapOffsetCheckbox.translatesAutoresizingMaskIntoConstraints = false
             overlapOffsetCheckbox.alignment = .left
 
+            let stackBadgeCheckbox = NSButton(checkboxWithTitle: NSLocalizedString("Show stacked window badge on hover", tableName: "Main", value: "", comment: ""), target: self, action: #selector(toggleStackBadge(_:)))
+            stackBadgeCheckbox.state = Defaults.stackBadge.userEnabled ? .on : .off
+            stackBadgeCheckbox.translatesAutoresizingMaskIntoConstraints = false
+            stackBadgeCheckbox.alignment = .left
+
             mainStackView.addArrangedSubview(gridHeaderLabel)
             mainStackView.setCustomSpacing(4, after: gridHeaderLabel)
             mainStackView.addArrangedSubview(showAdditionalSizesCheckbox)
             mainStackView.addArrangedSubview(overlapOffsetCheckbox)
-            mainStackView.setCustomSpacing(8, after: overlapOffsetCheckbox)
+            mainStackView.addArrangedSubview(stackBadgeCheckbox)
+            mainStackView.setCustomSpacing(8, after: stackBadgeCheckbox)
             mainStackView.addArrangedSubview(cyclingHintLabel)
             mainStackView.setCustomSpacing(8, after: cyclingHintLabel)
             mainStackView.addArrangedSubview(topLeftEighthRow)
@@ -978,6 +989,7 @@ class SettingsViewController: NSViewController {
                 widthStepField.trailingAnchor.constraint(equalTo: largerWidthShortcutView.trailingAnchor),
                 showAdditionalSizesCheckbox.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),
                 overlapOffsetCheckbox.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),
+                stackBadgeCheckbox.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),
                 smallerWidthShortcutView.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),
                 topVerticalThirdShortcutView.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),
                 middleVerticalThirdShortcutView.leadingAnchor.constraint(equalTo: largerWidthShortcutView.leadingAnchor),

Rectangle/StackBadge/StackBadgeGeometry.swift

@@ -0,0 +1,90 @@
+//
+//  StackBadgeGeometry.swift
+//  Rectangle
+//
+//  Copyright © 2026 Ryan Hanson. All rights reserved.
+//
+
+import Foundation
+
+/// Pure geometry for the stack badge: where on a screen a grid cell's
+/// top-left corner can be, and whether the cursor is resting near one.
+/// No AX, no window state - computable from screen frames alone.
+enum StackBadgeGeometry {
+
+    /// Every grid layout Rectangle can place windows into, as (columns, rows).
+    /// Cell corners from these are where stacked windows can share an origin.
+    static let gridDimensions: [(cols: Int, rows: Int)] = [
+        (2, 1), (1, 2),         // halves
+        (3, 1), (1, 3),         // thirds
+        (2, 2),                 // quarters
+        (4, 1), (1, 4),         // fourths
+        (3, 2), (2, 3),         // sixths
+        (4, 2),                 // eighths
+        (3, 3),                 // ninths
+        (4, 3), (3, 4),         // twelfths
+        (4, 4)                  // sixteenths
+    ]
+
+    /// Top-left corners (AppKit coordinates, y up) of every grid cell across
+    /// all supported grids, deduplicated. ~30 points for a typical screen.
+    static func cornerPoints(in screenFrame: CGRect) -> [CGPoint] {
+        guard screenFrame.width > 0, screenFrame.height > 0 else { return [] }
+        var points = [CGPoint]()
+        for grid in gridDimensions {
+            for col in 0..<grid.cols {
+                for row in 0..<grid.rows {
+                    let point = CGPoint(
+                        x: screenFrame.minX + CGFloat(col) * screenFrame.width / CGFloat(grid.cols),
+                        y: screenFrame.maxY - CGFloat(row) * screenFrame.height / CGFloat(grid.rows)
+                    )
+                    if !points.contains(where: { abs($0.x - point.x) < 1 && abs($0.y - point.y) < 1 }) {
+                        points.append(point)
+                    }
+                }
+            }
+        }
+        return points
+    }
+
+    /// The indices of the window origins (AX coordinates) that form a
+    /// cascade stack. The overlap offset cascades diagonally: +x always, but
+    /// y can run either way (the offset is applied in AppKit coordinates,
+    /// where +y converts to an upward move in AX space), so the y window is
+    /// symmetric. Every origin is tried as the stack's left anchor and the
+    /// densest cluster wins, so neither unrelated neighbors in a gap-widened
+    /// candidate box nor an unrelated leftmost outlier can distort the count.
+    static func stackIndices(among origins: [CGPoint], cascadeRange: CGFloat, tolerance: CGFloat) -> [Int] {
+        var best = [Int]()
+        for anchor in origins {
+            let cluster = origins.indices.filter { index in
+                let dx = origins[index].x - anchor.x
+                let dy = origins[index].y - anchor.y
+                return dx >= -tolerance && dx <= cascadeRange
+                    && dy >= -cascadeRange && dy <= cascadeRange
+            }
+            if cluster.count > best.count {
+                best = cluster
+            }
+        }
+        return best
+    }
+
+    /// The corner whose hover zone contains the point, or nil. The zone is a
+    /// square extending right and down from the corner (down in AppKit means
+    /// minus y), covering where gap-shifted windows and their title bars sit
+    /// relative to the geometric corner.
+    static func corner(near point: CGPoint, in corners: [CGPoint], zone: CGFloat) -> CGPoint? {
+        var best: (corner: CGPoint, distance: CGFloat)?
+        for corner in corners {
+            let dx = point.x - corner.x
+            let dy = corner.y - point.y
+            guard dx >= -4, dx <= zone, dy >= -4, dy <= zone else { continue }
+            let distance = dx * dx + dy * dy
+            if best == nil || distance < best!.distance {
+                best = (corner, distance)
+            }
+        }
+        return best?.corner
+    }
+}