UI Test overlapping trouble (in Build)

https://www.youtube.com/watch?v=3NMyj1_91FA&feature=youtu.be

As depicted in the video, the sentences at the top changes according to whichever button is pressed below. But somehow, the old sentences remain until I maximise then minimise the window.

What could be the cause? As I shouldn't have to do that every time the text changes.
Here's the code that control the texts.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class Storytelling : MonoBehaviour {
     public Text story, opt1, opt2, opt3;
     public int curStoProgressIndex, choice1, choice2, choice3;
     public string[] scenario; 
 
     // Use this for initialization
     void Start () {
         curStoProgressIndex = PlayerPrefs.GetInt ("CurrentPage", 0);
         Screen.fullScreen = false;
     }
     
     // Update is called once per frame
     void Update () {
         switch (curStoProgressIndex) {
         case 0:
             story.text = "Azaria has finally arrived home and when he is about to unlock the front door," +
                 "\n he forgot his key, but he....";
             ButtonSenarios("Kick the door down", 1, "Lick the keyhole", 2);
             break;
         case 1:
             story.text = "He tries to kick the door down, in fact, he kicks it soo hard that " +
                 "\n he bent his knee backward. He screams like a cat.";
             ButtonSenarios("He should cook his knee", 3, "He should unbent his knee", 4, "Do nothing about the knee", 5);
             break;
         case 2:
             story.text = "He suddenly remembers that he live in a gingerbread house. Thus no key required. " +
                 "\n he's in and he heads to.....";
             ButtonSenarios("Kicthen", 6, "Bedroom", 7, "Artic", 8);
             break;
         case 3:
             story.text = "He limbs into the kitchen and hack off his broken leg with a ________ knife";
             ButtonSenarios("butter", 9, "butcher", 10);
             break;
         case 4:
             story.text = "He tries to bend his leg back into how it was but he make it worse now he is inside a box";
             ButtonSenarios("What?", 11, "Why?", 12);
             break;
         case 5:
             story.text = "Ending A: the lone turret";
             ButtonSenarios();
             break;
         case 6:
             story.text = "What is he doing in the kitchen?";
             ButtonSenarios("head towards the sink", 13, "head towards the stove", 14, "head towards the fridge", 15);
             break;
         case 7:
             story.text = "What is he doing in the bedroom?";
             ButtonSenarios("Undress", 16, "Sleep", 17);
             break;
         case 8:
             story.text = "Ending B: He walks downstair toward the attic, in there she discovered that she is right at the front " +
                 "door of her house";
             ButtonSenarios();
             break;
         case 9:
             story.text = "He brings his broken leg toward his head and proceed to spread butter on it. He wolfs down " +
                 "his entire leg. He becomes a bit tired after that so he heads toward the...";
             ButtonSenarios("Beadroom", 7);
             break;
         case 10:
             story.text = "Ending C: With a mighty swing, at first nothing happens, then a gust of blood exploded " +
                 "from his lower back and his top half slide off";
             ButtonSenarios();
             break;
         case 11:
             story.text = "What? wha what? whaat what what waaht";
             ButtonSenarios("What?", 11);
             break;
         case 12:
             story.text = "ending D: He cheated on his wife......";
             ButtonSenarios();
             break;
         case 13:
             story.text = "Ending E: He sits on one of the sinks with his pant down, clutching his knee. He then proceeds to drop the " +
                 "kids off at the pool. However, the kids don't like the swimming pool he have chosen, so they decided to " +
                 "forcefully board the bus and out they comes through everywhere where there is a hole but not the hole that" +
                 " they comes out of......Azaria combusted.";
             ButtonSenarios();
             break;
         case 14:
             story.text = "He strips naked in front of the stove, then proceed to lay as flat as possible on the stove " +
                 "facing downward. He turns all the dials to full. At first nothing happens, " +
                 "then a sensation of overwhelming bliss wash over him.";
             ButtonSenarios("So he is cooked, so what?", 18, "he's still a bit raw", 19);
             break;
         case 15:
             story.text = "Enind F: He strolls into the fridge and stare through a plastic glass at the end of " +
                 "the refrigerator, at the other end of the glass are a bunch of giant children.";
             ButtonSenarios();
             break;
         case 16:
             story.text = "He takes off every garments on his body. " +
                 "He uses a knife to take off his birthday suit.";
             ButtonSenarios(";", 20, ":", 21);
             break;
         case 17:
             story.text = "............................................................................." +
                 "...................................................................................," +
                 ".............................................";
             ButtonSenarios("................", 17, "the next mo4ni g", 22);
             break;
         case 18:
             story.text = "Azaria opens the front door. Immediately some kind of roast arouse his senses, " +
                 "so she went straight to investigate the source of the smell. Soon he found herself standing in " +
                 "front of a stove with a charred human body. He ate the body and.....";
             ButtonSenarios("he stood in front of the stove", 14, "he stood in front of the stove", 
                            14, "he stood in front of the stove", 14);
             break;
         case 19:
             story.text = "Ending G: A giant knife went down on his naked body, with even slices on a bed of rice.";
             ButtonSenarios();
             break;
         case 20:
             story.text = "Ending H: He eat himself like an Oroboros";
             ButtonSenarios();
             break;
         case 21:
             story.text = "He bent over and inserted his head into the gaping hole between his legs. " +
                 "First his head went in, then his torso and finally the rest of the leg. " +
                 "Forming into a giant egg and be ready to be reborn. ";
             ButtonSenarios();
             break;
         case 22:
             story.text = "Ending I: tT NESAT04 M)Otring, Whet6nQ HE nENBOCME s3H3e, SH45 BN09ELCE h2e3";
             ButtonSenarios();
             break;
         }
     }
 
     void ButtonSenarios (string firstSena = null, int fs_index = 0, string secSena = null, int ss_index = 0,
                          string thridSena = null, int ts_index = 0) {
         opt1.text = firstSena;
         choice1 = fs_index;
         opt2.text = secSena;
         choice2 = ss_index;
         opt3.text = thridSena;
         choice3 = ts_index;
     }
 
     public void Button1Clicked () {
         curStoProgressIndex = choice1;
         story.text = "";
     }
 
     public void Button2Clicked () {
         curStoProgressIndex = choice2;
         story.text = "";
     }
 
     public void Button3Clicked () {
         curStoProgressIndex = choice3;
         story.text = "";
     }
 }