Inconsistent bottom border shadow visibility in commit description textarea

### The problem

I found a UI bug regarding the bottom border shadow in the commit description textarea. The shadow is supposed to indicate that there is more content to scroll down to. However, it behaves inconsistently:

1. False Positive at the Bottom: When adding newlines (Enter) and reaching the absolute bottom of the textarea, the bottom border shadow still appears, even though there is no more content below.

2. Inconsistent State Update: If I delete the existing newlines and then type them back exactly as they were, the shadow does not behave consistently (e.g., deleting 3 newlines and adding 3 back is required to make the shadow reappear).

### Release version

Version 3.5.5 (x64)

### Operating system

Windows 11

### Steps to reproduce the behavior

https://github.com/user-attachments/assets/4609f611-33ee-4439-ad45-fd8188083ff0

1. Go to any repository in GitHub Desktop.

2. Click on the commit description textarea (under the summary field).

3. Type multiple lines of text or press 'Enter' multiple times until the vertical scrollbar appears.

4. Scroll down to the absolute bottom (or keep pressing 'Enter' until you are at the bottom).

5. See error: The bottom border shadow is still visible even though there is no more content below.

6. Delete a few newlines and type them back again to see the shadow inconsistently disappear and reappear.

### Log files

_No response_

### Screenshots

_No response_

### Additional context

### Expected behavior
To ensure UI consistency, the behavior should be unified into one of the following two approaches:

- Approach 1: The bottom shadow should be hidden when the user reaches the absolute bottom of the textarea (indicating no more content to scroll down to).

- Approach 2: The bottom shadow should always be visible as long as the content overflows and a scrollbar exists, regardless of the current scroll position.

Currently, which feels like an error.

### Contribution
I would love to contribute by fixing this UI bug. Could you please let me know whether you prefer Approach 1, Approach 2, or if you have any other insights on how this should be handled? Once a direction is decided, I will investigate the components responsible for this textarea and submit a Pull Request.

Start by reproducing the behavior in the commit description textarea in GitHub Desktop using the listed steps and video. Confirm with maintainers whether the bottom shadow should track the absolute scroll position or remain visible whenever content overflows; done means the chosen behavior is consistent when adding, deleting, and restoring newlines.

Written by the indexing model from the issue text.