fix(window): display loaded buffer even if file got deactivated mid-yield

[dlyongemallo]

Fixes a crash that occurs if user hits <tab>/<s-tab> rapidly when diffview first opens.

[Copilot]

Pull request overview

Fixes an async race during initial Diffview open where rapid <tab>/<s-tab> navigation can deactivate an in-flight file between Layout.open_files’ load loop and open loop, leaving a window stuck on the null placeholder and causing downstream cursor-jump failures.

Changes:

• Update Window.open_file() to only gate loading on file.active, but still display an already-loaded buffer even if the file became inactive mid-yield.
• Add a functional regression test covering the “loaded-but-deactivated” window display case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lua/diffview/scene/window.lua	Adjusts Window.open_file() control flow to avoid leaving open_null displayed when the file buffer is already loaded but became inactive.
lua/diffview/tests/functional/window_spec.lua	Adds a regression test ensuring an already-loaded buffer is still displayed even if file.active=false.

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