Add hover background to split action buttons (#2271)

* Add hover background to split action buttons

Split buttons (terminal, browser, split right/down) now show a subtle
rounded-rect background highlight on hover. Matches standard macOS
toolbar button behavior.

* Prevent fade overlays from bleeding into bottom separator

* Render bottom separator above fade overlays to prevent bleed

* Exclude drop zone from scroll fade threshold

* Revert button hover, fix fade threshold with 32pt buffer

* Fix right fade threshold: subtract drop zone, 4pt tolerance

* Add leading padding to split buttons, fix fade threshold

* Rework tab bar: full-width scroll with floating split buttons

* Use ultraThinMaterial blur for floating split buttons

* Make split buttons group full height

* Full-height split button blur background

* Inset split buttons from bottom separator

* Fix blur overlapping separator

* Use matching tab bar bg for split buttons, clear separator

* Use regularMaterial blur for split buttons

* Try thickMaterial for split buttons

* Fade gradient + solid barFill for floating split buttons

* Use extended right fade as split buttons backdrop

* Add 5 debug styles for split button background

* Add Split Button Style debug window to Debug menu

* Clean up: no-bg floating buttons, alphabetical debug menu

- Split buttons float with no background (tabBarBackground covers the
  area, scroll padding prevents tabs from appearing behind buttons)
- Default splitButtonsWidth to 120 so first render has correct padding
- Remove split button style debug window and debug styles
- Alphabetize Debug Windows menu entries, remove dividers

* Revert to HStack sibling layout, add debug menu docs to CLAUDE.md

- Split buttons are HStack siblings of the ScrollView, not overlays.
  Single .background() on parent, no compositing mismatch.
- Alphabetize Debug Windows menu, remove dividers.
- Document Debug menu in CLAUDE.md.

* Add Split Button Layout debug window with 5 switchable approaches

* Fix fade gradient color to match tab bar background

* Add fade color debug window with 6 color options

* Use mask for scroll fades, fixes color mismatch

* Hide scroll fades in minimal mode unless hovering

* Remove split buttons from layout when hidden in minimal mode

* Always show fades, overlay buttons to prevent scroll jump

* Add hover-only mask fade behind split buttons

* Reduce button mask area to 90pt

* Animate button mask smoothly

* Fade entire button group together via opacity

* Add blur behind buttons with fade mask

* Use theme barBackground for button backdrop

* Blur + theme tint for button backdrop

* More tint (0.85), less blur

* Tint 0.2, clear bottom border

* Use terminal bg color, add scroll trailing padding for buttons

* Less blur, paneBackground at 0.75 opacity

* paneBackground at 0.9 opacity

* 0.97 opacity for button backdrop

* Test: fully opaque paneBackground

* Test: solid red backdrop

* Gradient + solid paneBackground backdrop, no mask

* Force opaque paneBackground for button backdrop

* Use barBackground for button backdrop

* Use terminal bg (paneBackground forced opaque)

* Pre-composite backdrop color for exact match

* Add 6 switchable backdrop styles in debug window

* Mask-based button area hiding, no backdrop color needed

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

CLAUDE.md

@@ -141,6 +141,14 @@ When adding a regression test for a bug fix, use a two-commit structure so CI pr
 
 This makes it visible in the GitHub PR UI (Commits tab, check statuses) that the test genuinely fails without the fix.
 
+## Debug menu
+
+The app has a **Debug** menu in the macOS menu bar (only in DEBUG builds). Use it for visual iteration:
+
+- **Debug > Debug Windows** contains panels for tuning layout, colors, and behavior. Entries are alphabetical with no dividers.
+- To add a debug toggle or visual option: create an `NSWindowController` subclass with a `shared` singleton, add it to the "Debug Windows" menu in `Sources/cmuxApp.swift`, and add a SwiftUI view with `@AppStorage` bindings for live changes.
+- When the user says "debug menu" or "debug window", they mean this menu, not `defaults write`.
+
 ## Pitfalls
 
 - **Custom UTTypes** for drag-and-drop must be declared in `Resources/Info.plist` under `UTExportedTypeDeclarations` (e.g. `com.splittabbar.tabtransfer`, `com.cmux.sidebar-tab-reorder`).

Sources/cmuxApp.swift

@@ -474,14 +474,12 @@ struct cmuxApp: App {
 
                 Divider()
                 Menu("Debug Windows") {
-                    Button("Debug Window Controls…") {
+                    Button("Background Debug…") {
-                        DebugWindowControlsWindowController.shared.show()
+                        BackgroundDebugWindowController.shared.show()
                     }
-
                     Button("Browser Import Hint Debug…") {
                         BrowserImportHintDebugWindowController.shared.show()
                     }
-
                     Button(
                         String(
                             localized: "debug.menu.browserProfilePopoverDebug",
@@ -490,26 +488,21 @@ struct cmuxApp: App {
                     ) {
                         BrowserProfilePopoverDebugWindowController.shared.show()
                     }
-
+                    Button("Debug Window Controls…") {
-                    Button("Settings/About Titlebar Debug…") {
+                        DebugWindowControlsWindowController.shared.show()
-                        SettingsAboutTitlebarDebugWindowController.shared.show()
                     }
-
-                    Divider()
-                    Button("Sidebar Debug…") {
-                        SidebarDebugWindowController.shared.show()
-                    }
-
-                    Button("Background Debug…") {
-                        BackgroundDebugWindowController.shared.show()
-                    }
-
                     Button("Menu Bar Extra Debug…") {
                         MenuBarExtraDebugWindowController.shared.show()
                     }
-
+                    Button("Settings/About Titlebar Debug…") {
-                    Divider()
+                        SettingsAboutTitlebarDebugWindowController.shared.show()
-
+                    }
+                    Button("Sidebar Debug…") {
+                        SidebarDebugWindowController.shared.show()
+                    }
+                    Button("Split Button Layout Debug…") {
+                        SplitButtonLayoutDebugWindowController.shared.show()
+                    }
                     Button("Open All Debug Windows") {
                         openAllDebugWindows()
                     }
@@ -3337,6 +3330,76 @@ private struct MenuBarExtraDebugView: View {
     }
 }
 
+// MARK: - Split Button Layout Debug Window
+
+private final class SplitButtonLayoutDebugWindowController: NSWindowController, NSWindowDelegate {
+    static let shared = SplitButtonLayoutDebugWindowController()
+
+    private init() {
+        let window = NSPanel(
+            contentRect: NSRect(x: 0, y: 0, width: 320, height: 240),
+            styleMask: [.titled, .closable, .utilityWindow],
+            backing: .buffered,
+            defer: false
+        )
+        window.title = "Split Button Layout"
+        window.titleVisibility = .visible
+        window.titlebarAppearsTransparent = false
+        window.isMovableByWindowBackground = true
+        window.isReleasedWhenClosed = false
+        window.identifier = NSUserInterfaceItemIdentifier("cmux.splitButtonLayoutDebug")
+        window.center()
+        window.contentView = NSHostingView(rootView: SplitButtonLayoutDebugView())
+        AppDelegate.shared?.applyWindowDecorations(to: window)
+        super.init(window: window)
+        window.delegate = self
+    }
+
+    @available(*, unavailable)
+    required init?(coder: NSCoder) { fatalError() }
+
+    func show() {
+        window?.center()
+        window?.makeKeyAndOrderFront(nil)
+    }
+}
+
+private struct SplitButtonLayoutDebugView: View {
+    @AppStorage("debugFadeColorStyle") private var backdropStyle = 0
+
+    private let options: [(Int, String)] = [
+        (0, "Pre-composited paneBackground"),
+        (1, "Raw paneBackground (opaque)"),
+        (2, "barBackground (tab chrome)"),
+        (3, "windowBackgroundColor"),
+        (4, "controlBackgroundColor"),
+        (5, "Pre-composited barBackground"),
+    ]
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 10) {
+            Text("Button Backdrop Color")
+                .font(.headline)
+
+            ForEach(options, id: \.0) { id, label in
+                HStack {
+                    Image(systemName: backdropStyle == id ? "checkmark.circle.fill" : "circle")
+                        .foregroundColor(backdropStyle == id ? .accentColor : .secondary)
+                    Text(label)
+                }
+                .contentShape(Rectangle())
+                .onTapGesture { backdropStyle = id }
+            }
+
+            Text("Changes apply live.")
+                .font(.caption)
+                .foregroundColor(.secondary)
+        }
+        .padding(16)
+        .frame(maxWidth: .infinity, alignment: .topLeading)
+    }
+}
+
 // MARK: - Background Debug Window
 
 private final class BackgroundDebugWindowController: NSWindowController, NSWindowDelegate {

vendor/bonsplit

@@ -1 +1 @@
-Subproject commit 447ac42b45256bdf333659d2dbe955afcaa87f6b
+Subproject commit aff9f7073125d412fed07bce1adab0e78f907b14