A scroll bar is like a 2d camera control that controlls the scroll area's viewport's contents. I will assume you know how to render your gui in its entirety. I also strongly advice against making a gui that is complex unless it truly contributes to ux. This could lead to a mediocre gaming experience. I also strongly urge you to use a UI library.

In abstract high level pseudo code:

1. Initialization:
2. Define the scrollable area's width & height.
3. Define the viewport's width & height.
4. The "handle" is the same % of the scrollbar as the viewport is of the scrollable plain.
   • The "handle" is the rectangle that travels up & down a vertical scrollbar.

Example: The scrollable area's height is 1000 pixels and the viewport's height is 200 pixels so the "handle" will be 20% of the scrollbar's height, if the scrollbar has the same height as the viewport than the handle will be 40 pixels high.

2. Main loop:

3. Render the entire scrollable area (if it's not gigantic).

   • Otherwise you need to smartly render only elements close the current position of the viewable area.

4. The "handle" has an upper and lower bounds in % of the scrollbar.

5. Blit the relevant rectangle with the same upper and lower % of the scrollable area.

6. Place the result inside the viewport.

7. Accepting input:

8. When a user clicks in the scrollable area's viewport, consider the "handle's" upper bound.

9. Adjust the click to a position % of the scrollable's area pixels lower based on upper bound.

10. Dragging the scrollbar's "hadnle":

11. Detect a mouseDown event that occurs on the scrollbar.

    • If the event occured on the handle:
      1. Remember the position.
      2. Adjust the handle and viewport with the correct offset from that position until the mouseUp event occurs.
    • If the event occurs on the scrollbar but not on the "handle":
      1. Adjust the "handle" so it's centered on that position.

The explanation is based on a vertical scrollbar but the same applies to a horizontal bar.

A scroll bar is like a 2D camera control that controls the scroll area's viewport's contents. I will assume you know how to render your gui in its entirety. I also strongly advice against making a gui that is complex unless it truly contributes to ux. This could lead to a mediocre gaming experience. I also strongly urge you to use a UI library.

In abstract high level pseudo code:

1. Initialization:
2. Define the scrollable area's width and height.
3. Define the viewport's width and height.
4. The "handle" is the same % of the scrollbar as the viewport is of the scrollable plain.
   • The "handle" is the rectangle that travels up and down a vertical scrollbar.

Example: The scrollable area's height is 1000 pixels and the viewport's height is 200 pixels so the "handle" will be 20% of the scrollbar's height, if the scrollbar has the same height as the viewport than the handle will be 40 pixels high.

2. Main loop:

3. Render the entire scrollable area (if it's not gigantic).

   • Otherwise you need to smartly render only elements close the current position of the viewable area.

4. The "handle" has an upper and lower bounds in % of the scrollbar.

5. Blit the relevant rectangle with the same upper and lower % of the scrollable area.

6. Place the result inside the viewport.

7. Accepting input:

8. When a user clicks in the scrollable area's viewport, consider the "handle's" upper bound.

9. Adjust the click to a position % of the scrollable's area pixels lower based on upper bound.

10. Dragging the scrollbar's "handle":

11. Detect a mouseDown event that occurs on the scrollbar.

    • If the event occured on the handle:
      1. Remember the position.
      2. Adjust the handle and viewport with the correct offset from that position until the mouseUp event occurs.
    • If the event occurs on the scrollbar but not on the "handle":
      1. Adjust the "handle" so it's centered on that position.

The explanation is based on a vertical scrollbar but the same applies to a horizontal bar.