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

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

@@ -95,6 +95,19 @@ 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
@@ -115,6 +128,10 @@ 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 +14,8 @@ 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 tabsFillSaveTask: Task<Void, Never>?
     @State private var scrollBar: DefaultsValueModel<Bool>
     @State private var copyOnSelect: DefaultsValueModel<Bool>
     @State private var autoResume: DefaultsValueModel<Bool>
@@ -31,6 +33,7 @@ public struct TerminalSection: View {
         self.catalog = catalog
         self.hostActions = hostActions
         _surfaceTabBarFont = State(initialValue: hostActions.surfaceTabBarFontSize())
+        _tabsFillPaneWidth = State(initialValue: hostActions.surfaceTabsFillPaneWidth())
         _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 +61,15 @@ public struct TerminalSection: View {
         }
     }
 
+    /// Persists the stretch-tabs-to-fill flag, cancelling any in-flight save so a
+    /// rapid toggle sequence only reflects the latest value.
+    private func saveTabsFillPaneWidth(_ enabled: Bool) {
+        tabsFillSaveTask?.cancel()
+        tabsFillSaveTask = Task {
+            _ = await hostActions.setSurfaceTabsFillPaneWidth(enabled)
+        }
+    }
+
     @ViewBuilder
     private var resumeCommandsCard: some View {
         SettingsCard {
@@ -129,6 +141,22 @@ 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.")
+            ) {
+                Toggle("", isOn: Binding(get: { tabsFillPaneWidth }, set: { newValue in
+                    tabsFillPaneWidth = newValue
+                    saveTabsFillPaneWidth(newValue)
+                }))
+                .labelsHidden()
+                .controlSize(.small)
+                .accessibilityIdentifier("SettingsTerminalTabsFillPaneWidthToggle")
+            }
+            SettingsCardDivider()
             SettingsCardRow(
                 configurationReview: .json("terminal.showScrollBar"),
                 String(localized: "settings.terminal.scrollBar", defaultValue: "Show Terminal Scroll Bar"),

Resources/Localizable.xcstrings

@@ -143076,6 +143076,57 @@
         }
       }
     },
+    "settings.terminal.tabsFillPaneWidth": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Stretch Tabs to Fill Pane Width"
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "タブをペーン幅いっぱいに広げる"
+          }
+        }
+      }
+    },
+    "settings.terminal.tabsFillPaneWidth.subtitleOff": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Tabs use a fixed width and scroll horizontally when they overflow the pane."
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "タブは固定幅を使用し、ペーンからあふれると横方向にスクロールします。"
+          }
+        }
+      }
+    },
+    "settings.terminal.tabsFillPaneWidth.subtitleOn": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "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."
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "タブが各ペーンのタブバーいっぱいに広がります。タブが1つのときは全幅に広がり、複数あるときは均等に分け合い、あふれたときだけスクロールします。"
+          }
+        }
+      }
+    },
     "settings.terminal.scrollBar": {
       "extractionState": "manual",
       "localizations": {

Sources/CmuxApplicationSupportDirectories.swift

@@ -187,6 +187,9 @@ enum CmuxGhosttyConfigSettingEditor {
     static let minSurfaceTabBarFontSize = 8.0
     static let maxSurfaceTabBarFontSize = 14.0
 
+    static let surfaceTabsFillPaneWidthKey = "surface-tabs-fill-pane-width"
+    static let defaultSurfaceTabsFillPaneWidth = false
+
     static func clampedSidebarFontSize(_ value: Double) -> Double {
         guard value.isFinite else { return defaultSidebarFontSize }
         return min(max(value, minSidebarFontSize), maxSidebarFontSize)
@@ -213,6 +216,35 @@ enum CmuxGhosttyConfigSettingEditor {
         parsedFontSize(in: contents, key: surfaceTabBarFontSizeKey, clamp: clampedSurfaceTabBarFontSize)
     }
 
+    /// Serializes a boolean Ghostty config value (`true`/`false`).
+    static func formattedBool(_ value: Bool) -> String {
+        value ? "true" : "false"
+    }
+
+    /// Reads the last occurrence of the surface-tabs-fill-pane-width flag, or `nil` if unset/unparseable.
+    static func parsedSurfaceTabsFillPaneWidth(in contents: String) -> Bool? {
+        guard let raw = parsedValue(for: surfaceTabsFillPaneWidthKey, in: contents) else {
+            return nil
+        }
+        return parsedBoolLiteral(raw)
+    }
+
+    /// Parses a Ghostty-style boolean literal, accepting the common truthy/falsy spellings.
+    static func parsedBoolLiteral(_ rawValue: String) -> Bool? {
+        let normalized = rawValue
+            .trimmingCharacters(in: CharacterSet(charactersIn: "\""))
+            .trimmingCharacters(in: .whitespaces)
+            .lowercased()
+        switch normalized {
+        case "true", "1", "yes", "on":
+            return true
+        case "false", "0", "no", "off":
+            return false
+        default:
+            return nil
+        }
+    }
+
     /// Formats a point size for display, trimming trailing zeros (`12`, `13.5`, `13.75`).
     static func formattedFontSize(_ value: Double) -> String {
         let scaled = Int((value * 100).rounded())

Sources/GhosttyConfig.swift

@@ -23,6 +23,7 @@ struct GhosttyConfig {
     var fontFamily: String = "Menlo"
     var fontSize: CGFloat = 12
     var surfaceTabBarFontSize: CGFloat = Self.defaultSurfaceTabBarFontSize
+    var surfaceTabsFillPaneWidth: Bool = CmuxGhosttyConfigSettingEditor.defaultSurfaceTabsFillPaneWidth
     var sidebarFontSize: CGFloat = Self.defaultSidebarFontSize
     var theme: String?
     var workingDirectory: String?
@@ -408,6 +409,10 @@ struct GhosttyConfig {
                     if let size = Double(value), size.isFinite {
                         surfaceTabBarFontSize = Self.clampedSurfaceTabBarFontSize(CGFloat(size))
                     }
+                case "surface-tabs-fill-pane-width":
+                    if let enabled = CmuxGhosttyConfigSettingEditor.parsedBoolLiteral(value) {
+                        surfaceTabsFillPaneWidth = enabled
+                    }
                 case "sidebar-font-size":
                     if let size = Double(value), size.isFinite {
                         sidebarFontSize = Self.clampedSidebarFontSize(CGFloat(size))

Sources/HostSettingsActions.swift

@@ -190,6 +190,24 @@ final class HostSettingsActions: SettingsHostActions {
         )
     }
 
+    func surfaceTabsFillPaneWidth() -> Bool {
+        // See ``sidebarFontSize()`` — uses the cached config to avoid main-actor disk I/O.
+        GhosttyConfig.load().surfaceTabsFillPaneWidth
+    }
+
+    func setSurfaceTabsFillPaneWidth(_ enabled: Bool) async -> Bool {
+        let formatted = CmuxGhosttyConfigSettingEditor.formattedBool(enabled)
+        guard await fontConfigWriter.write(
+            key: CmuxGhosttyConfigSettingEditor.surfaceTabsFillPaneWidthKey,
+            value: formatted
+        ) else {
+            hostSettingsLogger.warning("failed to persist surface-tabs-fill-pane-width")
+            return false
+        }
+        GhosttyApp.shared.reloadConfiguration(source: "settings.terminal.tabsFillPaneWidth")
+        return true
+    }
+
     func formattedFontSize(_ points: Double) -> String {
         CmuxGhosttyConfigSettingEditor.formattedFontSize(points)
     }

Sources/Workspace.swift

@@ -10680,10 +10680,18 @@ final class Workspace: Identifiable, ObservableObject {
         bonsplitAppearance(
             from: config.backgroundColor,
             backgroundOpacity: config.backgroundOpacity,
-            tabTitleFontSize: config.surfaceTabBarFontSize
+            tabTitleFontSize: config.surfaceTabBarFontSize,
+            tabWidthMode: Self.bonsplitTabWidthMode(for: config)
         )
     }
 
+    /// Maps the `surface-tabs-fill-pane-width` config flag to Bonsplit's tab-width mode.
+    nonisolated static func bonsplitTabWidthMode(
+        for config: GhosttyConfig
+    ) -> BonsplitConfiguration.Appearance.TabWidthMode {
+        config.surfaceTabsFillPaneWidth ? .fill : .fixed
+    }
+
     nonisolated static func usesSharedSurfaceBackdrop(defaults: UserDefaults = .standard) -> Bool {
         defaults.bool(forKey: "sidebarMatchTerminalBackground")
     }
@@ -10816,7 +10824,8 @@ final class Workspace: Identifiable, ObservableObject {
     private static func bonsplitAppearance(
         from backgroundColor: NSColor,
         backgroundOpacity: Double,
-        tabTitleFontSize: CGFloat = 11
+        tabTitleFontSize: CGFloat = 11,
+        tabWidthMode: BonsplitConfiguration.Appearance.TabWidthMode = .fixed
     ) -> BonsplitConfiguration.Appearance {
         let sharesWindowBackdrop = usesWindowRootTerminalBackdrop()
         let renderingMode = WindowAppearanceSnapshot.terminalRenderingMode(
@@ -10831,6 +10840,7 @@ final class Workspace: Identifiable, ObservableObject {
         return BonsplitConfiguration.Appearance(
             tabBarHeight: WindowChromeMetrics.bonsplitTabBarHeight,
             tabTitleFontSize: tabTitleFontSize,
+            tabWidthMode: tabWidthMode,
             splitButtonBackdropEffect: Self.bonsplitSplitButtonBackdropEffect(),
             splitButtonTooltips: Self.currentSplitButtonTooltips(),
             enableAnimations: false,
@@ -10851,15 +10861,18 @@ final class Workspace: Identifiable, ObservableObject {
             renderingMode: renderingMode
         )
         let nextTabTitleFontSize = config.surfaceTabBarFontSize
+        let nextTabWidthMode = Self.bonsplitTabWidthMode(for: config)
         let currentAppearance = bonsplitController.configuration.appearance
         let currentTabTitleFontSize = currentAppearance.tabTitleFontSize
+        let currentTabWidthMode = currentAppearance.tabWidthMode
         let colorsChanged = !Self.bonsplitChromeColorsEqual(
             currentAppearance.chromeColors,
             nextChromeColors
         )
         let sharedBackdropChanged = currentAppearance.usesSharedBackdrop != sharesWindowBackdrop
         let fontSizeChanged = abs(currentTabTitleFontSize - nextTabTitleFontSize) > 0.0001
-        let isNoOp = !colorsChanged && !sharedBackdropChanged && !fontSizeChanged
+        let tabWidthModeChanged = currentTabWidthMode != nextTabWidthMode
+        let isNoOp = !colorsChanged && !sharedBackdropChanged && !fontSizeChanged && !tabWidthModeChanged
 
         if GhosttyApp.shared.backgroundLogEnabled {
             GhosttyApp.shared.logBackground(
@@ -10886,6 +10899,9 @@ final class Workspace: Identifiable, ObservableObject {
         if fontSizeChanged {
             bonsplitController.configuration.appearance.tabTitleFontSize = nextTabTitleFontSize
         }
+        if tabWidthModeChanged {
+            bonsplitController.configuration.appearance.tabWidthMode = nextTabWidthMode
+        }
 
         if GhosttyApp.shared.backgroundLogEnabled {
             GhosttyApp.shared.logBackground(
@@ -10974,11 +10990,12 @@ final class Workspace: Identifiable, ObservableObject {
         // and keep split entry instantaneous.
         // Use the cached Ghostty config so new workspaces inherit tab-strip sizing
         // without paying repeated parse costs on the workspace-creation hot path.
-        let initialSurfaceTabBarFontSize = GhosttyConfig.load().surfaceTabBarFontSize
+        let initialConfig = GhosttyConfig.load()
         let appearance = Self.bonsplitAppearance(
             from: GhosttyApp.shared.defaultBackgroundColor,
             backgroundOpacity: GhosttyApp.shared.defaultBackgroundOpacity,
-            tabTitleFontSize: initialSurfaceTabBarFontSize
+            tabTitleFontSize: initialConfig.surfaceTabBarFontSize,
+            tabWidthMode: Self.bonsplitTabWidthMode(for: initialConfig)
         )
         let config = BonsplitConfiguration(
             allowSplits: true,