# Objective

- Fixes an issue where setting an arbitrarily high `Vertical Scroll
Margin` fails to perfectly lock the cursor in the center of the viewport
(Typewriter Mode?). In certain cases one could go up/down a line before
auto-scroll gets triggered, deviating from user intent.
- Fixes #63862

## Solution

- **The Issue:** When `Vertical Scroll Margin` is very high, the
effective margin applied above and below the cursor is derived from the
viewport's free space:
  ```rust
  ((visible_lines - (target_bottom - target_top)) / 2.0).floor()
  ```
- **The Fix:** Instead of using the same rounded number for both the top
and bottom margins, calculate them separately:
  1. The top margin stays the same as before.
2. The bottom margin is calculated by subtracting the top margin from
the total free screen space.
- This ensures the bottom margin automatically catches whatever fraction
the floor operation on the top margin drops, so the two margins always
sum to the full available space.

## Testing

- Did you test these changes? If so, how?
Ran `cargo test -p editor`, all passed. I've also attached the results
of manual testing in the Showcase section below.

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([[UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

Before and after screen recording

<details>
<summary>Click to view showcase</summary>

**Before:**


https://private-user-images.githubusercontent.com/63847494/646999418-4dd388b8-4aba-4004-ac4f-6f1e0dad0f70.mp4?jwt=eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.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.DG6RpiEM-781ZctlvtQeWhw3kAYJ4KS6rNsAmMU38fg

**After:**


https://github.com/user-attachments/assets/4bf870e0-84ba-449a-bf2a-925c4a90d5ed

</details>

Release Notes:

- Fixed the cursor not staying perfectly centered when using a high
Vertical Scroll Margin (Typewriter Mode).

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>