Single line on button press

#include "FastLED.h"
#define NUM_LEDS 64
#define PIN 2
#define BUTTON_PIN 4
CRGB leds [NUM_LEDS];
uint8_t gHue = 0; //rotating "base" color
bool buttonState = LOW;
bool lastButtonState = LOW;

void setup(){
pinMode(BUTTON_LEAD,INPUT_PULLUP);
FastLED.addLeds<WS2812B,PIN,GRB,>(leds,NUM_LEDS).setCorrection(TypicalLEDStrip);
FastLED.setBrightness(64);
}

void loop(){
if (buttonListener()){line();}
}

void fadeall(){for(int i=0; i<NUM_LEDS;i++){leds[i].nscale8(150);}}

void line(){
for(int i=0; i<NUM_LEDS+8; i++){
if(buttonListener()) {i=0;}
leds[i] = CHSV (gHue++,255,255);
FastLED.show();
fadeall();
delay(3);
}
}

bool buttonListener(){  //monitor button press
bool modeChanged = false;
buttonState = digitalRead(BUTTON_PIN);
if(buttonState != lastButtonState{ if (buttonState == LOW){
mode++ //probably not needed here
modeChanged = true;
delay(150);  //debouncing delay
}
}
lastButtonState = buttonState;
return modeChanged;
}