Fix lag when scrolling horizontally to seek, #4153

This commit will:
- Add methods displayActive and displayIdle to VideoView
- Add use of a Timer to delay stopping the display link
- Change MPVController to ensure display link is running while seeking
- Change PlayerCore to ensure display link is running while stepping
- Change MainWindowController to ensure display link is running when
  entering and leaving full screen mode

Rebased with develop and corrected merge conflicts.

iina/MPVController.swift

@@ -870,6 +870,12 @@ class MPVController: NSObject {
 
     case MPV_EVENT_SEEK:
       player.info.isSeeking = true
+      DispatchQueue.main.sync {
+        // When playback is paused the display link may be shutdown in order to not waste energy.
+        // It must be running when seeking to avoid slowdowns caused by mpv waiting for IINA to call
+        // mpv_render_report_swap.
+        player.mainWindow.videoView.displayActive()
+      }
       if needRecordSeekTime {
         recordedSeekStartTime = CACurrentMediaTime()
       }
@@ -882,6 +888,14 @@ class MPVController: NSObject {
     case MPV_EVENT_PLAYBACK_RESTART:
       player.info.isIdle = false
       player.info.isSeeking = false
+      DispatchQueue.main.sync {
+        // When playback is paused the display link may be shutdown in order to not waste energy.
+        // The display link will be restarted while seeking. If playback is paused shut it down
+        // again.
+        if player.info.isPaused {
+          player.mainWindow.videoView.displayIdle()
+        }
+      }
       if needRecordSeekTime {
         recordedSeekTimeListener?(CACurrentMediaTime() - recordedSeekStartTime)
         recordedSeekTimeListener = nil
@@ -1051,9 +1065,9 @@ class MPVController: NSObject {
             // the timer that synchronizes the UI and the high priority display link thread.
             if paused {
               player.invalidateTimer()
-              player.mainWindow.videoView.stopDisplayLink()
+              player.mainWindow.videoView.displayIdle()
             } else {
-              player.mainWindow.videoView.startDisplayLink()
+              player.mainWindow.videoView.displayActive()
               player.createSyncUITimer()
             }
           }

iina/MainWindowController.swift

@@ -1173,6 +1173,9 @@ class MainWindowController: PlayerWindowController {
   }
 
   func windowWillEnterFullScreen(_ notification: Notification) {
+    // When playback is paused the display link is stopped in order to avoid wasting energy on
+    // needless processing. It must be running while transitioning to full screen mode.
+    videoView.displayActive()
     if isInInteractiveMode {
       exitInteractiveMode(immediately: true)
     }
@@ -1234,8 +1237,15 @@ class MainWindowController: PlayerWindowController {
       fadeableViews.append(additionalInfoView)
     }
 
-    if Preference.bool(for: .playWhenEnteringFullScreen) && player.info.isPaused {
-      player.resume()
+    if player.info.isPaused {
+      if Preference.bool(for: .playWhenEnteringFullScreen) {
+        player.resume()
+      } else {
+        // When playback is paused the display link is stopped in order to avoid wasting energy on
+        // needless processing. It must be running while transitioning to full screen mode. Now that
+        // the transition has completed it can be stopped.
+        videoView.displayIdle()
+      }
     }
 
     if #available(macOS 10.12.2, *) {
@@ -1254,6 +1264,9 @@ class MainWindowController: PlayerWindowController {
   }
 
   func windowWillExitFullScreen(_ notification: Notification) {
+    // When playback is paused the display link is stopped in order to avoid wasting energy on
+    // needless processing. It must be running while transitioning from full screen mode.
+    videoView.displayActive()
     if isInInteractiveMode {
       exitInteractiveMode(immediately: true)
     }
@@ -1304,6 +1317,13 @@ class MainWindowController: PlayerWindowController {
       removeBlackWindow()
     }
 
+    if player.info.isPaused {
+      // When playback is paused the display link is stopped in order to avoid wasting energy on
+      // needless processing. It must be running while transitioning from full screen mode. Now that
+      // the transition has completed it can be stopped.
+      videoView.displayIdle()
+    }
+
     if #available(macOS 10.12.2, *) {
       player.touchBarSupport.toggleTouchBarEsc(enteringFullScr: false)
     }

iina/PlayerCore.swift

@@ -669,11 +669,18 @@ class PlayerCore: NSObject {
   }
 
   func frameStep(backwards: Bool) {
+    // When playback is paused the display link is stopped in order to avoid wasting energy on
+    // It must be running when stepping to avoid slowdowns caused by mpv waiting for IINA to call
+    // mpv_render_report_swap.
+    mainWindow.videoView.displayActive()
     if backwards {
       mpv.command(.frameBackStep)
     } else {
       mpv.command(.frameStep)
     }
+    if info.isPaused {
+      mainWindow.videoView.displayIdle()
+    }
   }
 
   func screenshot() {

iina/VideoView.swift

@@ -39,6 +39,8 @@ class VideoView: NSView {
 
   var pendingRedrawsAfterEnteringPIP = 0;
 
+  private var displayIdleTimer: Timer?
+
   lazy var hdrSubsystem = Logger.Subsystem(rawValue: "hdr")
 
   // MARK: - Attributes
@@ -203,6 +205,7 @@ class VideoView: NSView {
     guard let link = link else {
       Logger.fatal("Cannot Create display link!")
     }
+    guard !CVDisplayLinkIsRunning(link) else { return }
     updateDisplayLink()
     CVDisplayLinkSetOutputCallback(link, displayLinkCallback, mutableRawPointerOf(obj: player.mpv))
     CVDisplayLinkStart(link)
@@ -251,6 +254,38 @@ class VideoView: NSView {
     }
   }
 
+  // MARK: - Reducing Energy Use
+
+  /// Starts the display link if it has been stopped in order to save energy.
+  func displayActive() {
+    displayIdleTimer?.invalidate()
+    startDisplayLink()
+  }
+
+  /// Reduces energy consumption when the display link does not need to be running.
+  ///
+  /// Adherence to energy efficiency best practices requires that IINA be absolutely idle when there is no reason to be performing any
+  /// processing, such as when playback is paused. The [CVDisplayLink](https://developer.apple.com/documentation/corevideo/cvdisplaylink-k0k)
+  /// is a high-priority thread that runs at the refresh rate of a display. If the display is not being updated it is desirable to stop the
+  /// display link in order to not waste energy on needless processing.
+  ///
+  /// However, IINA will pause playback for short intervals when performing certain operations. In such cases it does not make sense to
+  /// shutdown the display link only to have to immediately start it again. To avoid this a `Timer` is used to delay shutting down the
+  /// display link. If playback becomes active again before the timer has fired then the `Timer` will be invalidated and the display link
+  /// will not be shutdown.
+  ///
+  /// - Note: In addition to playback the display link must be running for operations such seeking, stepping and entering and leaving
+  ///         full screen mode.
+  func displayIdle() {
+    displayIdleTimer?.invalidate()
+    // The time of 3 seconds is somewhat arbitrary. As mpv does not provide an event indicating a
+    // frame step has completed it must not be too short or will catch mpv still drawing when
+    // stepping.
+    displayIdleTimer = Timer.scheduledTimer(withTimeInterval: 3.0, repeats: false) { _ in
+      self.stopDisplayLink()
+    }
+  }
+
   func setICCProfile(_ displayId: UInt32) {
     if !Preference.bool(for: .loadIccProfile) {
       Logger.log("Not using ICC due to user preference", subsystem: hdrSubsystem)