Made Editor Separators Thicker (#2055)

### Description

Added a new split view separator style that results in a thicker separator.

### Related Issues

* #2054

### Checklist

- [x] I read and understood the [contributing guide](https://github.com/CodeEditApp/CodeEdit/blob/main/CONTRIBUTING.md) as well as the [code of conduct](https://github.com/CodeEditApp/CodeEdit/blob/main/CODE_OF_CONDUCT.md)
- [x] The issues this PR addresses are related to each other
- [x] My changes generate no new warnings
- [x] My code builds and runs on my machine
- [x] My changes are all related to the related issue above
- [x] I documented my code

### Screenshots

Before

<img width="1728" alt="image" src="https://github.com/user-attachments/assets/178f1457-9255-4948-b6c9-2ae1fecf5bc9" />

After

<img width="1728" alt="image" src="https://github.com/user-attachments/assets/ab9defef-2f8e-45c8-bce2-c2e8bcb41b2b" />

Author: austincondiff

CodeEdit/Features/Editor/Views/EditorLayoutView.swift

@@ -47,12 +47,14 @@ struct EditorLayoutView: View {
     }
 
     struct SubEditorLayoutView: View {
-        @ObservedObject var data: SplitViewData
+        @Environment(\.colorScheme)
+        private var colorScheme
 
+        @ObservedObject var data: SplitViewData
         @FocusState.Binding var focus: Editor?
 
         var body: some View {
-            SplitView(axis: data.axis) {
+            SplitView(axis: data.axis, dividerStyle: .editorDivider) {
                 splitView
             }
             .edgesIgnoringSafeArea([.top, .bottom])
@@ -61,12 +63,12 @@ struct EditorLayoutView: View {
         var splitView: some View {
             ForEach(Array(data.editorLayouts.enumerated()), id: \.offset) { index, item in
                 EditorLayoutView(layout: item, focus: $focus)
-                    .transformEnvironment(\.isAtEdge) { belowToolbar in
-                        calcIsAtEdge(current: &belowToolbar, index: index)
-                    }
-                    .environment(\.splitEditor) { [weak data] edge, newEditor in
-                        data?.split(edge, at: index, new: newEditor)
-                    }
+                   .transformEnvironment(\.isAtEdge) { belowToolbar in
+                       calcIsAtEdge(current: &belowToolbar, index: index)
+                   }
+                   .environment(\.splitEditor) { [weak data] edge, newEditor in
+                       data?.split(edge, at: index, new: newEditor)
+                   }
             }
         }
 

CodeEdit/Features/SplitView/Model/CodeEditDividerStyle.swift

@@ -0,0 +1,42 @@
+//
+//  CodeEditDividerStyle.swift
+//  CodeEdit
+//
+//  Created by Khan Winter on 5/30/25.
+//
+
+import AppKit
+
+/// The style of divider used by ``SplitView``.
+///
+/// To add a new style, add another case to this enum and fill in the ``customColor`` and ``customThickness``
+/// variables. When passed to ``SplitView``, the custom styles will be used instead of the default styles. Leave
+/// values as `nil` to use default styles.
+enum CodeEditDividerStyle: Equatable {
+    case system(NSSplitView.DividerStyle)
+    case editorDivider
+
+    var customColor: NSColor? {
+        switch self {
+        case .system:
+            return nil
+        case .editorDivider:
+            return NSColor(name: nil) { appearance in
+                if appearance.name == .darkAqua {
+                    NSColor.black
+                } else {
+                    NSColor(white: 203.0 / 255.0, alpha: 1.0)
+                }
+            }
+        }
+    }
+
+    var customThickness: CGFloat? {
+        switch self {
+        case .system:
+            return nil
+        case .editorDivider:
+            return 3.0
+        }
+    }
+}

CodeEdit/Features/SplitView/Views/SplitView.swift

@@ -9,19 +9,26 @@ import SwiftUI
 
 struct SplitView<Content: View>: View {
     var axis: Axis
+    var dividerStyle: CodeEditDividerStyle
     var content: Content
 
-    init(axis: Axis, @ViewBuilder content: () -> Content) {
+    init(axis: Axis, dividerStyle: CodeEditDividerStyle = .system(.thin), @ViewBuilder content: () -> Content) {
         self.axis = axis
+        self.dividerStyle = dividerStyle
         self.content = content()
     }
 
-    @State var viewController: () -> SplitViewController? = { nil }
+    @State private var viewController: () -> SplitViewController? = { nil }
 
     var body: some View {
         VStack {
             content.variadic { children in
-                SplitViewControllerView(axis: axis, children: children, viewController: $viewController)
+                SplitViewControllerView(
+                    axis: axis,
+                    dividerStyle: dividerStyle,
+                    children: children,
+                    viewController: $viewController
+                )
             }
         }
         ._trait(SplitViewControllerLayoutValueKey.self, viewController)

CodeEdit/Features/SplitView/Views/SplitViewControllerView.swift

@@ -10,15 +10,20 @@ import SwiftUI
 struct SplitViewControllerView: NSViewControllerRepresentable {
 
     var axis: Axis
+    var dividerStyle: CodeEditDividerStyle
     var children: _VariadicView.Children
     @Binding var viewController: () -> SplitViewController?
 
     func makeNSViewController(context: Context) -> SplitViewController {
-        context.coordinator
+        let controller = SplitViewController(axis: axis) { controller in
+            updateItems(controller: controller)
+        }
+        return controller
     }
 
     func updateNSViewController(_ controller: SplitViewController, context: Context) {
         updateItems(controller: controller)
+        controller.setDividerStyle(dividerStyle)
     }
 
     private func updateItems(controller: SplitViewController) {
@@ -61,40 +66,92 @@ struct SplitViewControllerView: NSViewControllerRepresentable {
     }
 
     func makeCoordinator() -> SplitViewController {
-        SplitViewController(parent: self, axis: axis)
+        SplitViewController(axis: axis, setUpItems: nil)
     }
 }
 
 final class SplitViewController: NSSplitViewController {
+    final class CustomSplitView: NSSplitView {
+        @Invalidating(.display)
+        var customDividerStyle: CodeEditDividerStyle = .system(.thin) {
+            didSet {
+                switch customDividerStyle {
+                case .system(let dividerStyle):
+                    self.dividerStyle = dividerStyle
+                case .editorDivider:
+                    return
+                }
+            }
+        }
+
+        init() {
+            super.init(frame: .zero)
+            dividerStyle = .thin
+        }
+
+        required init?(coder: NSCoder) {
+            fatalError("init(coder:) has not been implemented")
+        }
+
+        override var dividerColor: NSColor {
+            customDividerStyle.customColor ?? super.dividerColor
+        }
+
+        override var dividerThickness: CGFloat {
+            customDividerStyle.customThickness ?? super.dividerThickness
+        }
+    }
 
     var items: [SplitViewItem] = []
     var axis: Axis
-    var parentView: SplitViewControllerView
+    var parentView: SplitViewControllerView?
+
+    var setUpItems: ((SplitViewController) -> Void)?
 
-    init(parent: SplitViewControllerView, axis: Axis = .horizontal) {
+    init(axis: Axis, setUpItems: ((SplitViewController) -> Void)?) {
         self.axis = axis
-        self.parentView = parent
+        self.setUpItems = setUpItems
         super.init(nibName: nil, bundle: nil)
     }
 
     required init?(coder: NSCoder) {
         fatalError("init(coder:) has not been implemented")
     }
 
+    override func loadView() {
+        splitView = CustomSplitView()
+        super.loadView()
+    }
+
     override func viewDidLoad() {
+        super.viewDidLoad()
         splitView.isVertical = axis != .vertical
-        splitView.dividerStyle = .thin
+        setUpItems?(self)
         DispatchQueue.main.async { [weak self] in
-            self?.parentView.viewController = { [weak self] in
+            self?.parentView?.viewController = { [weak self] in
                 self
             }
         }
     }
 
-    override func splitView(_ splitView: NSSplitView, shouldHideDividerAt dividerIndex: Int) -> Bool {
+    override func splitView(_ splitView: NSSplitView, canCollapseSubview subview: NSView) -> Bool {
         false
     }
 
+    override func splitView(_ splitView: NSSplitView, shouldHideDividerAt dividerIndex: Int) -> Bool {
+        // For some reason, AppKit _really_ wants to hide dividers when there's only one item (and no dividers)
+        // so we do this check for them.
+        guard items.count > 1 else { return false }
+        return super.splitView(splitView, shouldHideDividerAt: dividerIndex)
+    }
+
+    func setDividerStyle(_ dividerStyle: CodeEditDividerStyle) {
+        guard let splitView = splitView as? CustomSplitView else {
+            return
+        }
+        splitView.customDividerStyle = dividerStyle
+    }
+
     func collapse(for id: AnyHashable, enabled: Bool) {
         items.first { $0.id == id }?.item.animator().isCollapsed = enabled
     }