F# VS editor: reduce background CPU load by gating expensive analyzers to the active document only

[xperiandri]

Is your feature request related to a problem? Please describe.

CPU profiling of devenv.exe (Visual Studio Insiders + F# extension) shows ThreadPoolWorkQueue.Dispatch at 72 % total / 59 % self, driven by a continuous stream of small tasks from FSharp.Compiler.CheckDeclarations, FSharpProjectOptionsReactor, and async/cancellableTask state-machine overhead.

The root cause is not a single hot function but the volume of background tasks scheduled regardless of whether an F# document is actually active/visible. Roslyn's default BackgroundAnalysisScope for C# is VisibleFilesAndOpenFilesWithPreviouslyReportedDiagnostics, but F# analyzers currently run a full typecheck for every open tab on every crawler pass.

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Describe the solution you'd like

Already implemented in this PR

A shared ActiveDocumentDetection module was extracted that uses SVsShellMonitorSelection / IVsMonitorSelection / IVsWindowFrame to determine whether a Roslyn Document is the currently focused editor tab. The module is now applied to the four heaviest analyzers/services:

Component	File	Fix applied
UnusedOpensDiagnosticAnalyzer	Diagnostics/UnusedOpensDiagnosticAnalyzer.fs	active-document gate (was already in place, refactored to shared module)
UnusedDeclarationsAnalyzer	Diagnostics/UnusedDeclarationsAnalyzer.fs	active-document gate added
SimplifyNameDiagnosticAnalyzer	Diagnostics/SimplifyNameDiagnosticAnalyzer.fs	active-document gate added
FSharpInlayHintsService	Hints/FSharpInlayHintsService.fs	active-document gate added

The shared helper (ActiveDocumentDetection.isActiveDocument) falls back to true (= don't suppress) when the active document cannot be determined, so analysis is never silently lost.

Remaining work (tracked in docs/ide/background-activity-minimization-plan.md)

#	Source	Estimated impact	Risk
4	ClassificationService.AddSemanticClassificationsAsync — cache miss triggers full typecheck for every open tab	Medium	Low-medium
5	createPEReference / Compilation.Emit — re-emits metadata for every C# referenced project on each workspace event	High	Medium
6	hasDependentVersionChanged — synchronous .Result blocks the reactor MailboxProcessor thread	Medium	Low
7	FSharpProjectOptionsReactor — FIFO queue starves active-document requests	Medium	Medium
8	Script updateProjectOptions fires on every caret move	Low-medium	Low
9	DocumentDiagnosticAnalyzer semantic path — no text-version short-circuit	Medium	Low
10	SymbolHelpers.findReferencedSymbolsAsync — typechecks all documents in solution	Low (on-demand)	Low

Full root-cause descriptions and proposed fixes are in docs/ide/background-activity-minimization-plan.md.

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Describe alternatives you've considered

• Lowering BackgroundAnalysisScope globally via IGlobalOptionService — blocked by internal Roslyn API; proposed as a future Roslyn API extension in docs/ide/api-designs/Active-Document-Background-Analysis.md.
• Using IDocumentTrackingService from Roslyn — not exposed via ExternalAccess; same future-API path.
• SemaphoreSlim throttling alone — already in place for SimplifyName but does not eliminate unnecessary work, just caps it.

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Additional context

• Profile was collected on Visual Studio 18.9 Insiders (devenv.exe) with the /rootsuffix RoslynDev hive.
• The immediate symptom is sluggish UI and continuous CPU consumption even when the user is not typing or switching tabs.
• The fix is fully backward-compatible: non-active documents receive fresh diagnostics/hints as soon as the user switches to them.