"Files changed" review comment gutter decorations disappear when collapsing or expanding a file

[smashwilson]

Prerequisites

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Description

The comment decorations in the gutter vanish from the "Files changed" tab when you collapse or expand any of the files within it.

Steps to Reproduce

1. Run the GitHub: Open Issue or Pull Request command from the command palette and enter the URL https://github.com/atom/github/pull/1882. Click "Open".
2. Navigate to the "Files Changed" tab on the item that opens. Note the "comment" icon the gutter.
3. Collapse the diff by clicking the down-arrow in the file header.
4. Expand the diff again.

Expected behavior:

Comment decorations within the collapsed file should disappear when the file they're on is collapsed, and be restored to their original positions when the file is re-expanded.

Actual behavior:

The comment decorations vanish. Decorations on files after a collapsed or expanded one seem to jump around as well.

Reproduces how often:

100%

Versions

atom/github@0.28.0

Additional Information

Likely some markers getting messed up by the buffer changes made by expanding and collapsing. Maybe we could move the comment decoration markers to a MarkerLayer tracked by a PatchBuffer?