Feature: quickfiler-itemviewer-coverage #456

• Work Mode: full-feature

Problem / Why

The ItemViewer form family under QuickFiler/Viewers/ is entirely invisible to coverage
measurement. ItemViewer is a partial type spread across six hand-written source files plus a
6,224-line generated designer file, and the single [ExcludeFromCodeCoverage] attribute on
ItemViewer.cs suppresses instrumentation for the whole type. The only member of the family that
is measured, ItemViewerExpanded.cs, sits at 39.0% line and 8.3% branch coverage - below both the
80% line and 75% branch gates that issue #136 and .claude/rules/general-unit-test.md set.

Under the epic's ratified policy reconciliation, [ExcludeFromCodeCoverage] on a testable seam is a
Blocking finding. The attribute on ItemViewer.cs has never been argued against the
irreducible-remainder standard, so the family is unratified exempt rather than legitimately exempt.

Proposed Behavior

Bring every testable file in the ItemViewer family to at least 80% line and 75% branch
coverage, verified with F1's per-file harness, by extracting host-neutral seams from the
UserControl-derived partials and removing the type-level [ExcludeFromCodeCoverage] attribute.
Classify IItemViewer.cs as interface-only / not-measured and the two *.Designer.cs files per
F1's ledger rules. No observable behavior change to QuickFiler flows.

Acceptance Criteria

• Every testable file in the family reaches >= 80% line and >= 75% branch coverage.
• The [ExcludeFromCodeCoverage] attribute on ItemViewer.cs is removed, or an irreducible
  remainder is ratified in F1's ledger with a file-specific rationale.
• IItemViewer.cs is reported N/A as interface-only / not-measured, with no attribute added.
• The two *.Designer.cs files are classified per the ledger's generated-code rules.
• No production file in scope exceeds 500 lines; created files reach >= 90% line coverage.
• Full C# toolchain green; repository-wide coverage retained or improved.

Constraints & Risks

• ItemViewer derives from UserControl, so the epic's STA last-resort clause for never-shown
  in-memory controls is available, but only after seam extraction has been exhausted.
• A partial type may carry [ExcludeFromCodeCoverage] on only one part; annotating two parts is
  CS0579. The designer partial therefore cannot be exempted independently of the hand-written
  partials once the attribute is removed.
• ItemViewer.WebViewThread.cs and ItemViewer.Breadcrumb.cs cross the WebView2 thread boundary
  and carry ordering invariants; tests must use an injected clock and fake timers.
• Sibling boundaries: drop-down/WebView2 host files belong to F13, breadcrumb bridge and messenger
  files to F12, QfcItemController.* to F10. None may be edited by this child.
• UtilitiesCS grants no InternalsVisibleTo to QuickFiler.Test; any seam must be local.

Test Conditions

• Unit coverage for command dispatch, display-state transitions, and folder-search filtering.
• Breadcrumb ordering invariants across the WebView2 thread boundary with a fake clock.
• Expanded-viewer population and teardown paths currently at 39.0% line / 8.3% branch.
• STA-scoped construction of the never-shown control, in a dedicated *.StaTests.cs file, only
  where no seam can isolate the logic.

Source

From: docs/features/potential/2026-08-07-quickfiler-itemviewer-coverage.md