float x = 0;
float w = 100;

void setup() {
  size(600,50);
  fill(255, 0, 0);
  stroke(0);
  rect(x, 0, w, height);
}

void draw() {
  background(0);
  if (mousePressed)
    if ((mouseButton == LEFT) && (mouseX >= x) && (mouseX <= x + w))
      x += mouseX - pmouseX;
  if (x > width - w) x = width - w;
  if (x < 0) x = 0;
  rect(x, 0, w, height);
}