Viewport scrolling results in rendering artifacts

**Describe the bug**
When I put a viewport next to a listview, and press `j` once, I get scrolling artifacts visible in the viewport.

**Setup**
Please complete the following information along with version numbers, if applicable.
- OS: macOS
- Shell: zsh
- Terminal Emulator: iTerm 2
- Terminal Multiplexer: None
- Locale: en_US.UTF-8

**To Reproduce**
Steps to reproduce the behavior:
1. Clone https://github.com/benwaffle/bubbletea-viewport-bug
2. Run `go run .`
3. Press `j`

**Source Code**
https://github.com/benwaffle/bubbletea-viewport-bug

**Expected behavior**
No rendering artifacts.

**Screenshots**

**Additional Details**
Changing the `panelStyle` margin to 0 seems to get rid of the issue.