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Jiří Polášek 4986915dae CmdPal: Batch ViewModel property change notifications (#44545 )

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## Summary of the Pull Request

This PR introduces batching for property change notifications emitted by
Command Palette view models. It also adds a secondary notification path
that is guaranteed to execute on a background thread.

- Introduces **`BatchUpdateManager`**, which batches
`INotifyPropertyChanged` events from view models and replays them in a
coordinated way.
- Slightly reduces UI thread contention and allows related UI updates to
be applied together, reducing visual "tearing" in list items (when
title, subtitle and icon are updated separately with slight delay).
Batching won't mitigate all occurences, but its good enough and works
auto-magically.
- Adds a complementary background notification event that:
  - Is guaranteed to run on a background thread.
  - Fires before UI-thread notifications.
- Allows consumers to attach handlers without blocking COM out-of-proc
objects.

- Updates `TopLevelViewModel` to subscribe to the background property
change event instead of the UI-thread one.
- This avoids unintentionally shifting work onto the UI thread and
re-triggering expensive operations there.
- Previously, because `TopLevelViewModel` wraps another view model and
our view models raise `INPC` on the UI thread by default, its handler
was executing on the UI thread and re-raising the event as
`IListItem.PropertyChanged`, causing `FetchProperty` methods to run on
the UI thread again.
- Ideally, `TopLevelViewModel` should be reworked to address this more
cleanly, but that turned out to be a non-trivial change. This PR applies
a targeted mitigation in the meantime.








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## PR Checklist

- [ ] Closes: #xxx
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and link it here: #xxx

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## Detailed Description of the Pull Request / Additional comments

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2026-01-28 21:56:11 -06:00