Custom editor (e.g. Markdown preview) missing bottom border

## Description

The bottom of a custom editor is missing its border. This was first observed in the Markdown editor (contributed by the built-in markdown-language-features extension), and the same issue was reproduced in the Agents Window, suggesting this is a broader custom editor rendering issue rather than something specific to the Markdown extension.

## Steps to Reproduce

1. Open a file that uses a custom editor (e.g. the Markdown editor).
2. Observe the bottom edge of the editor area.
3. Notice that the border present on other edges is missing at the bottom.

## Notes from discussion

- Reported by @tylerjlawrence, with a screenshot showing the missing border in the Markdown editor.
- @lramos15 confirmed the same missing border appears in the Agents Window, which points at a larger/shared issue rather than something markdown-specific.
- @hediet looked into it and noted: "The markdown editor is just a custom editor from the markdown extension. This seems to be a custom editor issue" (not owned by the chat/markdown extension team specifically).

## Suspected Area

Likely related to the shared custom editor webview/pane implementation (`src/vs/workbench/contrib/customEditor/`) rather than the markdown extension itself, given it reproduces in another surface (Agents Window) that also hosts a custom-editor-like view.