Add app.equalizeSplitsOnCreate to balance panes on new splits

Packages/macOS/CmuxPanes/Sources/CmuxPanes/Geometry/ExternalTreeNode+SplitGeometry.swift

@@ -28,6 +28,83 @@ extension ExternalTreeNode {
         )
     }
 
+    /// Plans an equalize pass over the run of same-orientation splits that
+    /// directly contains `paneId`: the pane's parent split plus every
+    /// ancestor reached without crossing a split of another orientation,
+    /// and the same-orientation splits nested inside that run. Splits
+    /// elsewhere in the tree, including same-orientation splits behind a
+    /// cross-orientation subtree, are left alone. Returns an empty plan when
+    /// `paneId` is absent or has no parent split.
+    public func equalizeDividerPlan(forSplitRunContainingPaneId paneId: String) -> SplitEqualizePlan {
+        guard let path = splitPath(toPaneId: paneId), let parent = path.last else {
+            return SplitEqualizePlan(adjustments: [], foundSplit: false, hadInvalidSplitIds: false)
+        }
+        var runRoot = parent
+        for ancestor in path.dropLast().reversed() {
+            guard ancestor.orientation == parent.orientation else { break }
+            runRoot = ancestor
+        }
+        var adjustments: [SplitDividerAdjustment] = []
+        var foundSplit = false
+        var hadInvalidSplitIds = false
+        ExternalTreeNode.split(runRoot).appendRunEqualizeAdjustments(
+            orientation: parent.orientation,
+            adjustments: &adjustments,
+            foundSplit: &foundSplit,
+            hadInvalidSplitIds: &hadInvalidSplitIds
+        )
+        return SplitEqualizePlan(
+            adjustments: adjustments,
+            foundSplit: foundSplit,
+            hadInvalidSplitIds: hadInvalidSplitIds
+        )
+    }
+
+    /// The splits from this node down to `paneId`'s parent, or `nil` when
+    /// the pane is not in this subtree.
+    private func splitPath(toPaneId paneId: String) -> [ExternalSplitNode]? {
+        switch self {
+        case .pane(let pane):
+            return pane.id == paneId ? [] : nil
+        case .split(let splitNode):
+            guard let rest = splitNode.first.splitPath(toPaneId: paneId)
+                ?? splitNode.second.splitPath(toPaneId: paneId) else { return nil }
+            return [splitNode] + rest
+        }
+    }
+
+    /// Post-order equalize over a same-orientation run: recurses only into
+    /// children that continue the run.
+    private func appendRunEqualizeAdjustments(
+        orientation: String,
+        adjustments: inout [SplitDividerAdjustment],
+        foundSplit: inout Bool,
+        hadInvalidSplitIds: inout Bool
+    ) {
+        guard case .split(let splitNode) = self, splitNode.orientation == orientation else { return }
+        splitNode.first.appendRunEqualizeAdjustments(
+            orientation: orientation,
+            adjustments: &adjustments,
+            foundSplit: &foundSplit,
+            hadInvalidSplitIds: &hadInvalidSplitIds
+        )
+        splitNode.second.appendRunEqualizeAdjustments(
+            orientation: orientation,
+            adjustments: &adjustments,
+            foundSplit: &foundSplit,
+            hadInvalidSplitIds: &hadInvalidSplitIds
+        )
+        foundSplit = true
+        guard let splitId = UUID(uuidString: splitNode.id) else {
+            hadInvalidSplitIds = true
+            return
+        }
+        let firstSpanCount = splitNode.first.spanCount(along: orientation)
+        let secondSpanCount = splitNode.second.spanCount(along: orientation)
+        let position = CGFloat(firstSpanCount) / CGFloat(firstSpanCount + secondSpanCount)
+        adjustments.append(SplitDividerAdjustment(splitId: splitId, position: position))
+    }
+
     private func appendEqualizeAdjustments(
         orientationFilter: String?,
         adjustments: inout [SplitDividerAdjustment],

Packages/macOS/CmuxPanes/Sources/CmuxPanes/PaneLayoutService.swift

@@ -22,7 +22,24 @@ public struct PaneLayoutService {
         controller: BonsplitController,
         orientationFilter: String? = nil
     ) -> SplitEqualizeResult {
-        let plan = node.equalizeDividerPlan(orientationFilter: orientationFilter)
+        apply(node.equalizeDividerPlan(orientationFilter: orientationFilter), to: controller)
+    }
+
+    /// Equalizes only the run of same-orientation splits that directly
+    /// contains `paneId` (see
+    /// ``ExternalTreeNode/equalizeDividerPlan(forSplitRunContainingPaneId:)``),
+    /// so a new split rebalances its own row or column without resetting
+    /// dividers elsewhere in the tree.
+    @discardableResult
+    public func equalizeSplitRun(
+        containingPaneId paneId: String,
+        in node: ExternalTreeNode,
+        controller: BonsplitController
+    ) -> SplitEqualizeResult {
+        apply(node.equalizeDividerPlan(forSplitRunContainingPaneId: paneId), to: controller)
+    }
+
+    private func apply(_ plan: SplitEqualizePlan, to controller: BonsplitController) -> SplitEqualizeResult {
         var allSucceeded = !plan.hadInvalidSplitIds
         for adjustment in plan.adjustments {
             if !controller.setDividerPosition(adjustment.position, forSplit: adjustment.splitId, fromExternal: true) {

Packages/macOS/CmuxPanes/Tests/CmuxPanesTests/SplitGeometryTests.swift

@@ -126,6 +126,48 @@ struct SplitGeometryTests {
         #expect(plan.adjustments.isEmpty)
     }
 
+    @Test func splitRunPlanRebalancesTheNewPanesRow() {
+        let outerId = UUID()
+        let innerId = UUID()
+        let tree = split(
+            outerId,
+            orientation: "horizontal",
+            first: pane("a"),
+            second: split(innerId, orientation: "horizontal", first: pane("b"), second: pane("c"))
+        )
+
+        let plan = tree.equalizeDividerPlan(forSplitRunContainingPaneId: "c")
+
+        #expect(plan.foundSplit)
+        #expect(plan.adjustments.map(\.splitId) == [innerId, outerId])
+        #expect(plan.adjustments[0].position == 0.5)
+        #expect(abs(plan.adjustments[1].position - (1.0 / 3.0)) < 0.0001)
+    }
+
+    @Test func splitRunPlanLeavesOtherRowsAndOrientationsAlone() {
+        let rootId = UUID()
+        let topRowId = UUID()
+        let bottomRowId = UUID()
+        let tree = split(
+            rootId,
+            orientation: "vertical",
+            dividerPosition: 0.3,
+            first: split(topRowId, orientation: "horizontal", dividerPosition: 0.2, first: pane("a"), second: pane("b")),
+            second: split(bottomRowId, orientation: "horizontal", dividerPosition: 0.5, first: pane("c"), second: pane("d"))
+        )
+
+        let plan = tree.equalizeDividerPlan(forSplitRunContainingPaneId: "d")
+
+        #expect(plan.adjustments.map(\.splitId) == [bottomRowId])
+        #expect(plan.adjustments[0].position == 0.5)
+    }
+
+    @Test func splitRunPlanIsEmptyWithoutAParentSplit() {
+        #expect(!pane("solo").equalizeDividerPlan(forSplitRunContainingPaneId: "solo").foundSplit)
+        let tree = split(UUID(), orientation: "horizontal", first: pane("a"), second: pane("b"))
+        #expect(tree.equalizeDividerPlan(forSplitRunContainingPaneId: "missing").adjustments.isEmpty)
+    }
+
     // MARK: Resize planning
 
     @Test func resizeMovesControllingDividerByPixelDelta() {

Packages/macOS/CmuxSettings/Sources/CmuxSettings/Keys/AppCatalogSection.swift

@@ -93,6 +93,15 @@ public struct AppCatalogSection: SettingCatalogSection {
         userDefaultsKey: "focusHistoryIncludesPanesAndTabs"
     )
 
+    /// Whether creating a split rebalances the panes along the new split's
+    /// axis so they share the space evenly. Defaults to off, which keeps the
+    /// halve-the-source-pane behavior.
+    public let equalizeSplitsOnCreate = DefaultsKey<Bool>(
+        id: "app.equalizeSplitsOnCreate",
+        defaultValue: false,
+        userDefaultsKey: "equalizeSplitsOnCreate"
+    )
+
     public let preferredEditor = DefaultsKey<String>(
         id: "app.preferredEditor",
         defaultValue: "",

Packages/macOS/CmuxSettings/Tests/CmuxSettingsTests/SettingCatalogTests.swift

@@ -103,6 +103,10 @@ struct SettingCatalogTests {
         #expect(ids.contains("browser.defaultZoomLevel"))
     }
 
+    @Test func equalizeSplitsOnCreateDefaultsOff() {
+        #expect(!SettingCatalog().app.equalizeSplitsOnCreate.defaultValue)
+    }
+
     @Test func focusHistoryDefaultsToWorkspacesOnly() {
         #expect(!SettingCatalog().app.focusHistoryIncludesPanesAndTabs.defaultValue)
     }

Packages/macOS/CmuxSettingsUI/Sources/CmuxSettingsUI/Navigation/CuratedSettingEntry+Default.swift

@@ -43,6 +43,12 @@ extension Array where Element == CuratedSettingEntry {
                 title: String(localized: "settings.app.focusHistoryIncludesPanesAndTabs", defaultValue: "Include Panes and Tabs in Focus History"),
                 synonyms: "app.focusHistoryIncludesPanesAndTabs focus history back forward panes tabs workspaces only navigation"
             ),
+            .init(
+                section: .app,
+                id: "equalize-splits-on-create",
+                title: String(localized: "settings.app.equalizeSplitsOnCreate", defaultValue: "Equalize Splits on Create"),
+                synonyms: "app.equalizeSplitsOnCreate equalize balance distribute even equal split pane size new split"
+            ),
             .init(section: .app, id: "file-drops", title: String(localized: "settings.app.fileDrop.defaultBehavior", defaultValue: "File Drops"), synonyms: "File Drops drag drop files finder path text terminal editor split preview shift"),
             .init(section: .app, id: "preferred-editor", title: String(localized: "settings.app.preferredEditor", defaultValue: "Open Files With"), synonyms: "Open Files With app.preferredEditor editor open file code vscode visual studio zed sublime subl cursor"),
             .init(section: .app, id: "supported-file-previews", title: String(localized: "settings.app.openSupportedFilesInCmux", defaultValue: "Open Supported Files in cmux"), synonyms: "Open Supported Files in cmux app.openSupportedFilesInCmux cmd click file preview pdf image video audio quicklook quick look editor external"),

Packages/macOS/CmuxSettingsUI/Sources/CmuxSettingsUI/Sections/AppSection.swift

@@ -31,6 +31,7 @@ public struct AppSection: View {
     @State private var keepWorkspaceOpen: DefaultsValueModel<Bool>
     @State private var firstClick: DefaultsValueModel<Bool>
     @State private var focusHistoryIncludesPanesAndTabs: DefaultsValueModel<Bool>
+    @State private var equalizeSplitsOnCreate: DefaultsValueModel<Bool>
     @State private var fileDrop: DefaultsValueModel<FileDropDefaultBehavior>
     @State private var preferredEditor: DefaultsValueModel<String>
     @State private var openSupported: DefaultsValueModel<Bool>
@@ -96,6 +97,7 @@ public struct AppSection: View {
         _keepWorkspaceOpen = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.keepWorkspaceOpenWhenClosingLastSurface))
         _firstClick = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.focusPaneOnFirstClick))
         _focusHistoryIncludesPanesAndTabs = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.focusHistoryIncludesPanesAndTabs))
+        _equalizeSplitsOnCreate = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.equalizeSplitsOnCreate))
         _fileDrop = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.fileDropDefaultBehavior))
         _preferredEditor = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.preferredEditor))
         _openSupported = State(initialValue: DefaultsValueModel(store: defaultsStore, key: catalog.app.openSupportedFilesInCmux))
@@ -158,7 +160,7 @@ public struct AppSection: View {
             mainCard
         }
         .task {
-            startSettingsObservation([language, appearance, appIcon, placement, inheritDir, minimalMode, keepWorkspaceOpen, firstClick, focusHistoryIncludesPanesAndTabs, fileDrop, preferredEditor, openSupported, openMarkdown, globalFontMagnification, markdownFontSize, markdownFontFamily, markdownMaxWidth, canvasPaneGap, canvasSnapping, fileEditorWordWrap, fileEditorSyntaxHighlighting, fileEditorLineNumbers, fileEditorIndentGuides, fileEditorCurrentLineHighlight, fileEditorTabWidth, iMessage, reorder, dockBadge, menuBarOnly, showInMenuBar, paneRing, paneFlash, desktopNotifications, agentPermissionPrompt, agentTurnComplete, agentIdleReminder, soundName, soundCommand, customSoundFile, soundOverrides, telemetry, confirmQuit, warnCloseTab, warnCloseX, hideCloseButton, renameSelects, paletteAllSurfaces])
+            startSettingsObservation([language, appearance, appIcon, placement, inheritDir, minimalMode, keepWorkspaceOpen, firstClick, focusHistoryIncludesPanesAndTabs, equalizeSplitsOnCreate, fileDrop, preferredEditor, openSupported, openMarkdown, globalFontMagnification, markdownFontSize, markdownFontFamily, markdownMaxWidth, canvasPaneGap, canvasSnapping, fileEditorWordWrap, fileEditorSyntaxHighlighting, fileEditorLineNumbers, fileEditorIndentGuides, fileEditorCurrentLineHighlight, fileEditorTabWidth, iMessage, reorder, dockBadge, menuBarOnly, showInMenuBar, paneRing, paneFlash, desktopNotifications, agentPermissionPrompt, agentTurnComplete, agentIdleReminder, soundName, soundCommand, customSoundFile, soundOverrides, telemetry, confirmQuit, warnCloseTab, warnCloseX, hideCloseButton, renameSelects, paletteAllSurfaces])
             if soundAgents.isEmpty {
                 soundAgents = await hostActions.notificationSoundAgentOptions()
             }
@@ -344,6 +346,24 @@ public struct AppSection: View {
             }
             SettingsCardDivider()
 
+            // Equalize Splits on Create
+            SettingsCardRow(
+                configurationReview: .json("app.equalizeSplitsOnCreate"),
+                String(localized: "settings.app.equalizeSplitsOnCreate", defaultValue: "Equalize Splits on Create"),
+                subtitle: equalizeSplitsOnCreate.current
+                    ? String(localized: "settings.app.equalizeSplitsOnCreate.subtitleOn", defaultValue: "New splits resize the panes in that direction to equal sizes.")
+                    : String(localized: "settings.app.equalizeSplitsOnCreate.subtitleOff", defaultValue: "New splits take half of the pane they were created from.")
+            ) {
+                Toggle("", isOn: Binding(
+                    get: { equalizeSplitsOnCreate.current },
+                    set: { equalizeSplitsOnCreate.set($0) }
+                ))
+                .labelsHidden()
+                .controlSize(.small)
+                .accessibilityIdentifier("SettingsEqualizeSplitsOnCreateToggle")
+            }
+            SettingsCardDivider()
+
             // File Drops
             SettingsCardRow(
                 configurationReview: .settingsOnly,

Packages/macOS/CmuxSettingsUI/Tests/CmuxSettingsUITests/SettingsRowAnchorResolutionTests.swift

@@ -38,6 +38,7 @@ struct SettingsRowAnchorResolutionTests {
     static let rowConfigPaths: [String] = [
         "app.commandPaletteSearchesAllSurfaces",
         "app.confirmQuit",
+        "app.equalizeSplitsOnCreate",
         "app.focusPaneOnFirstClick",
         "app.focusHistoryIncludesPanesAndTabs",
         "app.globalFontMagnification",