BEFORE
==================================
uniform sampler2D tex;
uniform float height;
uniform float width;
uniform float scrollbar_width;
uniform float offset_bottom;
uniform float offset_top;
uniform bool rtl;

void main ()
{
 vec4 color = cogl_color_in * texture2D (tex, vec2 (cogl_tex_coord_in[0].xy));
 float y = height * cogl_tex_coord_in[0].y;
 float x = width * cogl_tex_coord_in[0].x;
 float ratio = 0.0;
 float fade_bottom_start = height - offset_bottom;
 
 if (offset_top != 0.0 && y < offset_top && ((rtl && x > scrollbar_width) || (!rtl && x < (width - scrollbar_width)))) {
  ratio = y / offset_top;
  cogl_color_out = color * ratio;
 }
 else if (offset_bottom != 0.0 && y > fade_bottom_start && ((rtl && x > scrollbar_width) || (!rtl && x < (width - scrollbar_width)))) {
  ratio = (height - y)/(height - fade_bottom_start);
  cogl_color_out = color * ratio;
 }
 else { 
  cogl_color_out = color;
 }
}
================================
AFTER
===============================
uniform sampler2D tex;
uniform float height;
uniform float width;
uniform float scrollbar_width;
uniform float offset_bottom;
uniform float offset_top;
uniform bool rtl;

void main ()
{
 vec4 color = cogl_color_in * texture2D (tex, vec2 (cogl_tex_coord_in[0].xy));
 float y = height * cogl_tex_coord_in[0].y;
 float x = width * cogl_tex_coord_in[0].x;
 float ratio = 1.0;
 float fade_bottom_start = height - offset_bottom;
 float ratioA = y / offset_top;
 float ratioB = (height - y)/(height - fade_bottom_start);
 bool in_scroll_area = ((rtl && x > scrollbar_width) || (!rtl && x < (width - scrollbar_width)));
 bool fade_top = y < offset_top && in_scroll_area;
 bool fade_bottom = y > fade_bottom_start && in_scroll_area;
	    
 if (fade_top) {
  ratio = ratioA;
 }
 else if (fade_bottom) {
  ratio = ratioB;
 }
 cogl_color_out = color * ratio;
}