Add opt-in setting to stretch surface tabs to fill pane width (#5091)

Packages/CmuxSettingsUI/Sources/CmuxSettingsUI/Environment/SettingsHostActions.swift

@@ -100,6 +100,19 @@ public protocol SettingsHostActions: AnyObject {
     @discardableResult
     func setSurfaceTabBarFontSize(_ points: Double) async -> Bool
 
+    /// Whether surface tabs stretch to fill their pane's available tab-bar
+    /// width. Backed by the Ghostty config file (`surface-tabs-fill-pane-width`).
+    func surfaceTabsFillPaneWidth() -> Bool
+
+    /// Persists the surface-tabs-fill-pane-width flag to the Ghostty config and
+    /// live-reloads open windows.
+    ///
+    /// - Returns: `true` if the value was written and reloaded, `false` if
+    ///   persistence failed. The disk write happens off the main actor, so this
+    ///   is `async`; call it from a `Task` in the toggle action.
+    @discardableResult
+    func setSurfaceTabsFillPaneWidth(_ enabled: Bool) async -> Bool
+
     /// Formats a point size for display next to a font-size slider
     /// (e.g. `12`, `13.5`), trimming trailing zeros.
     func formattedFontSize(_ points: Double) -> String
@@ -172,6 +185,10 @@ public extension SettingsHostActions {
 
     func setSurfaceTabBarFontSize(_ points: Double) async -> Bool { true }
 
+    func surfaceTabsFillPaneWidth() -> Bool { false }
+
+    func setSurfaceTabsFillPaneWidth(_ enabled: Bool) async -> Bool { true }
+
     func formattedFontSize(_ points: Double) -> String {
         let scaled = (points * 100).rounded()
         let whole = Int(scaled / 100)

Packages/CmuxSettingsUI/Sources/CmuxSettingsUI/Sections/TerminalSection.swift

@@ -14,6 +14,11 @@ public struct TerminalSection: View {
     @State private var surfaceTabBarFont: SettingsFontSize
     @State private var fontSaveFailed = false
     @State private var fontSaveTask: Task<Void, Never>?
+    @State private var tabsFillPaneWidth: Bool
+    @State private var tabsFillPaneWidthLastSaved: Bool
+    @State private var tabsFillSaveFailed = false
+    @State private var tabsFillSaveTask: Task<Void, Never>?
+    @State private var tabsFillSaveGeneration = 0
     @State private var scrollBar: DefaultsValueModel<Bool>
     @State private var copyOnSelect: DefaultsValueModel<Bool>
     @State private var autoResume: DefaultsValueModel<Bool>
@@ -31,6 +36,9 @@ public struct TerminalSection: View {
         self.catalog = catalog
         self.hostActions = hostActions
         _surfaceTabBarFont = State(initialValue: hostActions.surfaceTabBarFontSize())
+        let initialTabsFillPaneWidth = hostActions.surfaceTabsFillPaneWidth()
+        _tabsFillPaneWidth = State(initialValue: initialTabsFillPaneWidth)
+        _tabsFillPaneWidthLastSaved = State(initialValue: initialTabsFillPaneWidth)
         _scrollBar = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.terminal.showScrollBar))
         _copyOnSelect = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.terminal.copyOnSelect))
         _autoResume = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.terminal.autoResumeAgentSessions))
@@ -58,6 +66,32 @@ public struct TerminalSection: View {
         }
     }
 
+    /// Persists the stretch-tabs-to-fill flag in request order while cancelling
+    /// superseded saves that have not started writing yet.
+    private func saveTabsFillPaneWidth(_ enabled: Bool) {
+        let previousSaveTask = tabsFillSaveTask
+        previousSaveTask?.cancel()
+        tabsFillSaveGeneration += 1
+        let saveGeneration = tabsFillSaveGeneration
+        tabsFillSaveFailed = false
+        tabsFillSaveTask = Task {
+            if let previousSaveTask {
+                await previousSaveTask.value
+            }
+            guard !Task.isCancelled else { return }
+            let saved = await hostActions.setSurfaceTabsFillPaneWidth(enabled)
+            if saved {
+                tabsFillPaneWidthLastSaved = enabled
+                if saveGeneration == tabsFillSaveGeneration {
+                    tabsFillSaveFailed = false
+                }
+            } else if saveGeneration == tabsFillSaveGeneration && !Task.isCancelled {
+                tabsFillPaneWidth = tabsFillPaneWidthLastSaved
+                tabsFillSaveFailed = true
+            }
+        }
+    }
+
     @ViewBuilder
     private var resumeCommandsCard: some View {
         SettingsCard {
@@ -129,6 +163,33 @@ public struct TerminalSection: View {
                 }
             }
             SettingsCardDivider()
+            SettingsCardRow(
+                configurationReview: .settingsOnly,
+                String(localized: "settings.terminal.tabsFillPaneWidth", defaultValue: "Stretch Tabs to Fill Pane Width"),
+                subtitle: tabsFillPaneWidth
+                    ? String(localized: "settings.terminal.tabsFillPaneWidth.subtitleOn", defaultValue: "Tabs stretch to fill each pane's tab bar. A single tab spans the full width; multiple tabs share it evenly and scroll only when they overflow.")
+                    : String(localized: "settings.terminal.tabsFillPaneWidth.subtitleOff", defaultValue: "Tabs use a fixed width and scroll horizontally when they overflow the pane."),
+                controlWidth: 250
+            ) {
+                VStack(alignment: .trailing, spacing: 4) {
+                    Toggle("", isOn: Binding(get: { tabsFillPaneWidth }, set: { newValue in
+                        tabsFillPaneWidth = newValue
+                        saveTabsFillPaneWidth(newValue)
+                    }))
+                    .labelsHidden()
+                    .controlSize(.small)
+                    .accessibilityIdentifier("SettingsTerminalTabsFillPaneWidthToggle")
+
+                    if tabsFillSaveFailed {
+                        Text(String(localized: "settings.terminal.tabsFillPaneWidth.saveFailed", defaultValue: "Couldn't save tab stretch setting. Please try again."))
+                            .font(.caption)
+                            .foregroundStyle(.red)
+                            .multilineTextAlignment(.trailing)
+                            .fixedSize(horizontal: false, vertical: true)
+                    }
+                }
+            }
+            SettingsCardDivider()
             SettingsCardRow(
                 configurationReview: .json("terminal.showScrollBar"),
                 String(localized: "settings.terminal.scrollBar", defaultValue: "Show Terminal Scroll Bar"),