Populate StubItem panes that are deserialized after package initialization

[smashwilson]

Please be sure to read the contributor's guide to the GitHub package before submitting any pull requests.

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• Suggestion: You can use checklists to keep track of progress for the sections on metrics, tests, documentation, and user research.

Description of the Change

We use StubItems to temporarily deserialize React-managed pane items, like the git or GitHub tabs, before the React tree is initialized and actually renders DOM elements within them to make them useful. The <PaneItem> component handles the logic of finding and "hydrating" any stubs that were deserialized from persisted workspace state on Atom launch. However, it's possible that StubItems will be deserialized directly into the workspace after the package is initialized, and <PaneItem> will never find them!

As a result, a user who opens a project after Atom has been launched - for example, from the File -> Reopen Project menu - will be stuck with blank stub panels until they close and re-open the items.

To fix this, let's watch newly added pane items on the workspace, detect when they're stubs that we don't already own, and hydrate them anyway.

Screenshot/Gif

Before:

After:

(TBD)

Alternate Designs

I'd love to revisit the way that StubItems work... I think we could simplify that way that they're created and managed to have a single "Item" class that's used consistently regardless of whether the item was opened through the <PaneItem>-registered opener function or deserialized from project state. That's a heavier change though and I feel like it'd risk introducing more regressions than it would fix, so I'm preferring the band-aid fix for now.

Benefits

At least one class of scenarios that would result in orphaned, blank StubItems will be patched.

Possible Drawbacks

We'll need to do more work on each pane item opening. This could result in performance impacts when large numbers of items are open, although I'll try to avoid it.

Applicable Issues

Fixes #2414.

Metrics

N/A

Tests

Following the verification steps in the linked issue, and adding unit tests.

Documentation

N/A

Release Notes

• Fixed a bug that would result in the Git and GitHub tabs being blank when deserialized from project state.

User Experience Research (Optional)

N/A

[codecov]

Codecov Report

Merging #2415 into master will decrease coverage by 0.00%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master    #2415      +/-   ##
==========================================
- Coverage   93.42%   93.41%   -0.01%     
==========================================
  Files         235      235              
  Lines       13147    13163      +16     
  Branches     1885     1889       +4     
==========================================
+ Hits        12282    12296      +14     
- Misses        865      867       +2     

Impacted Files	Coverage Δ
lib/atom/gutter.js	90.47% <0.00%> (-2.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1f9c0af...01791d1. Read the comment docs.