Keep Peek Definition off the main thread

[xperiandri]

Peek Definition on an F# symbol whose definition lives in metadata deadlocks Visual Studio — the whole
IDE, not just the editor. The easiest way to see it is to Peek twice: the first Peek from real source
usually resolves in source and is fine, and the second one, invoked from inside the metadata window
the first produced, hangs, because from there every symbol is external.

It presents as whatever the user happened to be doing at the time. It was reported to me as "rename
hangs"; the stack said Peek.

What the stack shows

Breaking into a hung instance, the UI thread reads, innermost last:

WindowFrame.Exec
└ PeekDefinitionCommandHandler.TriggerPeekDefinitionAtCaret
  └ PeekBroker.TriggerPeekSessionImpl → PeekSession.QueryPeekableItemSources
    └ PeekableItemSource.AugmentPeekSession
      └ VSUIThreadOperationExecutor.Execute
        └ JoinableTask.CompleteOnCurrentThread → WaitSynchronously → Task.Wait
          └ NoMessagePumpSyncContext.Wait

AugmentPeekSession runs on the main thread and holds it in JoinableTaskFactory.Run for the whole
augmentation, and that wait does not pump messages — which is why the whole IDE freezes. Meanwhile a
background thread sits in ThreadHelper.InvokeOnUIThread → ServiceProvider.QueryService, asking for
the thread that is waiting on it, and no thread is in FSharp.Compiler.Service.dll at all: the work is
not slow, it is stopped. Resuming does not break the cycle.

F# is reached from there through FSharpNavigableItemsService, the INavigableItemsService for F#
(F# documents have no semantic model, so Peek takes that path), which calls
IFSharpFindDefinitionService. Producing a definition that only exists as generated metadata then
asks for the main thread twice: TryGetExternalDeclarationAsync switches to it to open the document,
and creating the workspace project context underneath is itself a JoinableTaskFactory.Run.

The change

FSharpNavigation.FindDefinitionsAsync was serving both entry points — Go To Definition through
IFSharpGoToDefinitionService and Peek through IFSharpFindDefinitionService — so this splits it.
Go To Definition keeps the metadata case, since it owns the wait it makes. Peek gets
FindDefinitionsWithoutMetadataAsync, which stops at definitions that already have a document and
therefore needs the main thread nowhere.

Peek into metadata shows nothing until that is restored properly. That is a smaller regression than
freezing the IDE, and Go To Definition still opens the generated signature.

Why not just make it work

Roslyn's own metadata Peek asks for the main thread nowhere: DefinitionPeekableItem generates the
file in IPeekResultSource.FindResults, which the broker calls on a background thread ("we must block
the thread since the API doesn't support proper asynchrony"), and hands Peek a file path through
PeekHelpers.CreateDocumentPeekResult rather than opening a document.

Matching that from F# needs the project context to be creatable off the main thread, which is
dotnet/roslyn#85219, and a way to reach the generated document without OpenDocumentViaProject.
Neither can land here first.

Base

Stacked on #20482, which is the other main-thread fix in this area; the diff will shrink to its own
commit once that merges.

🤖 Generated with Claude Code

[github-actions]

❗ Release notes required

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---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`vsintegration/src`	docs/release-notes/.VisualStudio/18.vNext.md

[github-actions]

🔍 Tooling Safety Check — Affects-Design-Time
Affects-Design-Time: Visual Studio navigation code runs in the IDE.

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