Feature: efcviewer-breadcrumb-webview2

[drmoisan]

• Work Mode: full-feature

Problem / Why

The EfcViewer matching-folders control currently renders folder suggestions as a conventional
indented multi-row tree using BrightIdeasSoftware.TreeListView (QuickFiler/Viewers/EfcViewer.cs,
QuickFiler/Viewers/EfcViewer3.cs, plus their Designer files). The intended design is a single-line
breadcrumb per suggestion anchored at the selected leaf. The current hierarchy is synthesized by
FolderSuggestionTree.BuildFromRows via prefix-matching over the top-ranked suggestion rows, so
expanding a folder does not reveal its real Outlook subfolders. The prediction percentage is also
reported as obscured at runtime even though static column/rect math shows no overlap.

TreeListView (as currently used) does not naturally support single-line breadcrumb rendering with
per-segment double-click collapse, and it is a VSTO/WinForms-hosting-specific investment that would
not carry forward to the planned VSTO migration. The redesign targets WebView2 (HTML/CSS/JS), which
is largely reusable across a post-VSTO UI stack and reuses a dependency already proven in this
codebase (QuickFiler's WebView2 message-body pane, including the cid: fix from feature 326).

Proposed Behavior

Replace the EfcViewer matching-folders TreeListView in both EfcViewer and EfcViewer3 with a
WebView2-hosted HTML/CSS/JS breadcrumb control that:

1. Renders each suggestion as a single-line breadcrumb Folder -> SubFolder -> Leaf anchored at the
   selected/predicted leaf.
2. Shows an expand affordance (plus when collapsed, minus when expanded) only on the leaf, and only
   when the leaf has subfolders.
3. Collapses a row on double-click of a non-leaf segment, hiding everything after that segment and
   showing a plus to re-expand back to the full breadcrumb.
4. On expand of a segment, lists every real immediate Outlook subfolder of that folder via the
   shared live folder-hierarchy provider from feature 9101 (not prefix-matching over suggestion rows).
5. Keeps the prediction percentage always fully visible: first capture a runtime reproduction of the
   current obscuring defect, then apply a CSS-based fix.

A JS<->.NET event bridge carries double-click and keyboard (left/right arrow) interaction and routes
the live subfolder query across the WebView2 boundary. The percentage plumbing (feature 324) is reused
as-is; the scoring/ranking algorithm is not changed.

Acceptance Criteria

• Each EfcViewer suggestion renders as a single-line breadcrumb anchored at the selected leaf in a WebView2-hosted control (both EfcViewer and EfcViewer3).
• The leaf shows a plus/minus expand affordance only when it has subfolders.
• Double-clicking a non-leaf segment collapses the row after that segment with a plus to re-expand.
• Expanding a segment lists every real immediate Outlook subfolder via the 9101 provider.
• The prediction percentage is always fully visible; a runtime reproduction of the current defect precedes the CSS-based fix.
• A JS<->.NET bridge carries double-click and left/right-arrow keyboard events and routes the live subfolder query.
• No third-party WinForms tree/list control and no WPF/ElementHost are introduced; the control is WebView2 (HTML/CSS/JS).
• No change to the scoring/ranking algorithm; the C# toolchain (csharpier, analyzers, nullable, MSTest) is green and coverage thresholds are met.

Constraints & Risks

• Depends on issue 9101 (live Outlook folder-hierarchy provider), which is merged before this feature
  during epic execution. This feature consumes 9101's contract (ancestor chain + on-demand real
  immediate subfolders behind an injectable seam) rather than re-deriving hierarchy from suggestion rows.
• Ruled out: BrightIdeasSoftware.TreeListView (or any third-party WinForms tree/list control) and
  WPF/ElementHost.
• The live Outlook subfolder query is I/O-bound and must be isolated from pure breadcrumb logic per
  the repository I/O-boundary policy so the core is unit-testable without a live Outlook process.
• Two parallel non-shared EfcViewer implementations (EfcViewer, EfcViewer3) must both be converted.
• WinForms/WebView2 host wiring and Designer-generated code are coverage-exempt per policy; testable
  pure logic (breadcrumb model, bridge message shaping) is not exempt and must meet coverage floors.

Test Conditions

• Unit coverage: breadcrumb model construction from an ancestor chain; collapse/expand state transitions; bridge message serialization/deserialization; segment-children request shaping.
• Integration scenarios: WebView2 host initialization; JS<->.NET bridge round-trip for double-click, arrow keys, and subfolder query.
• Runtime reproduction of the percentage-obscuring defect and verification of the CSS fix.

Source

From: docs/features/potential/2026-07-16-efcviewer-breadcrumb-webview2.md