tab titles are relative to cwd if prefixes match

Tabs/Sources/Tabs/Tab.swift

@@ -22,12 +22,31 @@ final class Tab<Rep: TabRepresentative>: NSView {
       self.adjustToSelectionChange(newValue.isSelected)
     }
     didSet {
-      if self.titleView.stringValue == self.title { return }
+      updateTitle(self.title)
-      self.titleView.stringValue = self.title
-      self.adjustWidth()
     }
   }
 
+  private func updateTitle(_ title: String) {
+    let display_title = displayTitle(title)
+    if self.titleView.stringValue == display_title { return }
+    self.titleView.stringValue = display_title
+    self.adjustWidth()
+  }
+
+  private func displayTitle(_ title: String) -> String {
+    var display_title = title
+    if self.tabBar?.cwd != nil {
+      let cwd = ((self.tabBar?.cwd!)! as NSString).abbreviatingWithTildeInPath
+      let pref = title.commonPrefix(with: cwd)
+      if pref == cwd {
+        let cwd_component_count = (cwd as NSString).pathComponents.count
+        display_title = NSString.path(
+          withComponents: Array((title as NSString).pathComponents[cwd_component_count...]))
+      }
+    }
+    return display_title
+  }
+
   init(withTabRepresentative tabRepresentative: Rep, in tabBar: TabBar<Rep>) {
     self.tabBar = tabBar
     self.tabRepresentative = tabRepresentative
@@ -50,6 +69,10 @@ final class Tab<Rep: TabRepresentative>: NSView {
     self.adjustColors(self.isSelected)
   }
 
+  func updateContext() {
+    updateTitle(self.title)
+  }
+
   override func mouseUp(with _: NSEvent) { self.tabBar?.select(tab: self) }
 
   override func draw(_: NSRect) {

Tabs/Sources/Tabs/TabBar.swift

@@ -15,6 +15,11 @@ public final class TabBar<Rep: TabRepresentative>: NSView {
   public typealias TabCallback = (Int, Rep, [Rep]) -> Void
 
   public var theme: Theme { self._theme }
+  public var cwd: String? {
+    didSet {
+      self.tabs.forEach { $0.updateContext() }
+    }
+  }
 
   public var closeHandler: TabCallback?
   public var selectHandler: TabCallback?

VimR/VimR/MainWindow.swift

@@ -344,6 +344,7 @@ final class MainWindow: NSObject,
 
         if let cwd = state.cwdToSet {
           self.neoVimView.cwd = cwd
+          self.neoVimView.tabBar?.cwd = cwd.path
         }
 
         Completable