Drag edge of rectangle

OpenGL OpenGL: Basic Coding
1

I need to implement a rectangular slider for a program I’m working on.
Right now, I draw a rectangle using glRectf() at the bottom of the screen and I change the bottom-right coordiates depending on the mouse position if the mouse is inside the rectangle.
I have a global variable for the rectangle’s bottom right x position that changes in the mouse function. But, the rectangle always remain the same.
The relevant code is below.

void drawSlider()
{
	glColor3f(1.0f, 1.0f, 1.0f);
	glOrtho(0.1f, 500, 500, 0.0f, 0.0f, 1.0);
	glRectf(0.5f, 490.0f, sliderX, 500.0f);
}

void display()
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();  // reset the current matrix to identity matrix

// create a viewing matrix defined by an eye point and a reference point
glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

glEnable(GL_DEPTH_TEST);

	// Draw all components
	glPushMatrix();
		glRotatef(rotate_x, 1.0f, 0.0f, 0.0f);
		glRotatef(rotate_y, 0.0f, 1.0f, 0.0f);
		drawAxis();

		glScalef(scale, scale, scale);
		drawCube();
	glPopMatrix();

	drawSlider();

	glFlush();
	glutSwapBuffers();
}
void mouse(int button, int state, int x, int y)
{
	printf("Coordinates: (%d, %d)\n", x, y);

if ((button == GLUT_LEFT_BUTTON) && (state == GLUT_DOWN))
{
	if (y > 490.0f)
	{
		// Redraw rectangle
		sliderX = (float)x;
	}
	glutPostRedisplay();
}

I only check for top coordinate because the rectangle’s height is fixed and its always on the bottom of the (500x500) window.
Only the width should change on click and drag.
I haven’t added the code to the motion function because the mouse function itself is not working

Any tips on how to fix it?