Read a script's caret without waiting for the UI thread

[xperiandri]

Description

Fixes #20522

The project options reactor looked up the caret of a script or F# Miscellaneous Files document itself: ServiceProvider.GlobalProvider, the RDT, IVsTextManager.GetActiveView, IVsTextView.GetCaretPos, and an IVsTextViewEvents connection point. That code came with #18393. All of it needs the UI thread, so when the UI thread synchronously waits on project options, as breakpoint validation does when a document frame is shown, the reactor waits for the UI thread and the UI thread waits for the reactor. The reactor is a MailboxProcessor, so its work is not joined to the UI thread's JoinableTask, and moving the lookup to the UI thread with SwitchToMainThreadAsync would hang the same way.

Now the UI thread publishes the caret and the reactor only reads it:

• FocusedCaretTracker, an IWpfTextViewCreationListener for F# editors, keeps a FocusedCaret in each text buffer's properties. It holds the caret of whichever editor on the buffer has focus (ValueNone while none has), and raises LineChanged when the caret moves to another line or focus enters or leaves the buffer's editors. These are the same moments that OnChangeCaretLine, OnKillFocus and OnSetFocus reported before.
• The reactor reaches the FocusedCaret through the document's SourceText container, which does not need the UI thread. It passes the caret to GetProjectOptionsFromScript and re-submits the script options on LineChanged, as it did on the text view events before.
• Only scripts look for the caret. FCS reads it only in ScriptClosure.resolveDependencyManagerSources, to skip the #r "nuget: …" line being typed, and only scripts have those lines. A plain .fs file in F# Miscellaneous Files (the file that hung in VS hangs: project options reactor reads the caret through UI-thread COM while the UI thread waits for the reactor #20522) no longer subscribes to caret moves, and no longer recomputes script options on every caret line change.
• TryGetIVsTextView, TryGetTextViewAndCaretPos, subscribeToTextViewEvents, TextViewEventsHandler and ConnectionPointSubscription had no other callers and are removed with the opens that only they needed.

One behaviour follows the description of #18393 more closely than its code did. Moving focus to another window now submits the #r "nuget: …" line: without focus there is no caret. Before, GetActiveView(0, …) returned the last active view whether or not it had focus, so the line stayed excluded.

Checklist

• Test cases added. The hang needs the VS UI thread blocked in JoinableTaskFactory.Run; the editor tests have no UI thread and a null GlobalProvider, which is why they never hit it.
• Release notes entry updated

🤖 Generated with Claude Code

[github-actions]

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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-Restore
Affects-Design-Time: Changes IDE project-options code running during document activity.
Affects-Restore: Changes handling of active #r nuget package directives.

Generated by PR Tooling Safety Check · gpt56 1.3M · ◷

[T-Gro]

🤖 🕵️ LGTM