GH-812 Allow all fonts, but warn when variable width font is selected

NvimView/Sources/NvimView/NvimView.swift

@@ -91,8 +91,8 @@ public class NvimView: NSView,
     }
 
     set {
-      guard newValue.isFixedPitch else {
-        return
+      if !newValue.fontDescriptor.symbolicTraits.contains(.monoSpace) {
+        self.log.info("\(newValue) is not monospaced.")
       }
 
       let size = newValue.pointSize

VimR/VimR/AppDelegate.swift

@@ -60,6 +60,8 @@ class AppDelegate: NSObject, NSApplicationDelegate, NSUserNotificationCenterDele
     self.useSnapshot = initialAppState.useSnapshotUpdate
 
     super.init()
+
+    NSUserNotificationCenter.default.delegate = self
   }
 
   override func awakeFromNib() {

VimR/VimR/MainWindow.swift

@@ -503,6 +503,7 @@ class MainWindow: NSObject,
 
   private func showInitError() {
     let notification = NSUserNotification()
+    notification.identifier = UUID().uuidString
     notification.title = "Error during initialization"
     notification.informativeText =
     """

VimR/VimR/PrefMiddleware.swift

@@ -3,7 +3,7 @@
  * See LICENSE
  */
 
-import Foundation
+import Cocoa
 import DictionaryCoding
 import os
 
@@ -30,10 +30,33 @@ class PrefMiddleware: MiddlewareType {
     return { tuple in
       let result = reduce(tuple)
 
-      guard tuple.modified else {
+      guard result.modified else {
         return result
       }
 
+      let newFont = result.state.mainWindowTemplate.appearance.font
+      let traits = newFont.fontDescriptor.symbolicTraits
+
+      if newFont != self.currentFont {
+        self.currentFont = newFont
+
+        let newFontNameText: String
+        if let newFontName = newFont.displayName {
+          newFontNameText = ", \(newFontName),"
+        } else {
+          newFontNameText = ""
+        }
+
+        if !traits.contains(.monoSpace) {
+          let notification = NSUserNotification()
+          notification.identifier = UUID().uuidString
+          notification.title = "No monospaced font"
+          notification.informativeText = "The font you selected\(newFontNameText) does not seem "
+          + "to be a monospaced font. The rendering will most likely be broken."
+          NSUserNotificationCenter.default.deliver(notification)
+        }
+      }
+
       do {
         let dictionary: [String: Any] = try dictEncoder.encode(result.state)
         defaults.set(dictionary, forKey: PrefMiddleware.compatibleVersion)
@@ -45,8 +68,9 @@ class PrefMiddleware: MiddlewareType {
     }
   }
 
-  private let log = OSLog(subsystem: Defs.loggerSubsystem,
-                          category: Defs.LoggerCategory.middleware) 
+  private let log = OSLog(subsystem: Defs.loggerSubsystem, category: Defs.LoggerCategory.middleware)
+
+  private var currentFont = NSFont.userFixedPitchFont(ofSize: 13)!
 
   class MainWindowMiddleware: MiddlewareType {