This is a simple scroll bar that you can use in your projects. All you need to do is define the distance that you want the scroll bar to move things, and it will return a variable that you can use to adjust the y position of items, strings, etc.
Sprites can be changed and/or resized, and the scroll bar will adjust accordingly. The scroll bar handle size takes the width of the bounding box and it's height adjusts automatically to distance that it has to move things. You can adjust, reverse, or disable the mouse wheel speed, the arrow key speed, and the arrow button speed. You can change the speed that the handle comes to the cursor when you click on just the bar. You can adjust the minimum size of the handle, and how quickly the minimum size is reached as your range of scrolling increases.
The extension is explained in detail, and there are many helpful notes in the code to help you implement this scroll bar without too much hassle.
Made with GMS:2, entirely coded with GML. Tested on Windows.