perf: Only set the timers on UITextDisplayer when there are cues (#8114)

Author: avelad

lib/text/ui_text_displayer.js

@@ -170,6 +170,9 @@ shaka.text.UITextDisplayer = class {
         this.cues_.push(cue);
       }
     }
+    if (this.cues_.length) {
+      this.configureCaptionsTimer_();
+    }
 
     this.updateCaptions_();
   }
@@ -232,6 +235,9 @@ shaka.text.UITextDisplayer = class {
     // displaying if removed (say, due to the user changing languages).
     const forceUpdate = oldNumCues > this.cues_.length;
     this.updateCaptions_(forceUpdate);
+    if (!this.cues_.length) {
+      this.configureCaptionsTimer_();
+    }
 
     return true;
   }
@@ -259,11 +265,15 @@ shaka.text.UITextDisplayer = class {
    */
   configureCaptionsTimer_() {
     if (this.captionsTimer_) {
-      const captionsUpdatePeriod = this.config_ ?
-          this.config_.captionsUpdatePeriod : 0.25;
-      const updateTime = captionsUpdatePeriod /
-          Math.max(1, Math.abs(this.video_.playbackRate));
-      this.captionsTimer_.tickEvery(updateTime);
+      if (this.cues_.length) {
+        const captionsUpdatePeriod = this.config_ ?
+            this.config_.captionsUpdatePeriod : 0.25;
+        const updateTime = captionsUpdatePeriod /
+            Math.max(1, Math.abs(this.video_.playbackRate));
+        this.captionsTimer_.tickEvery(updateTime);
+      } else {
+        this.captionsTimer_.stop();
+      }
     }
   }