Feature: quickfiler-breadcrumb-webview2

[drmoisan]

• Work Mode: full-feature

Problem / Why

The folder-tree-percentage-ui epic (issues 324, 325, 326, 327) delivered a QuickFiler folder
dropdown whose behavior does not match the intended design. The dropdown is still a stock
System.Windows.Forms.ComboBox (CboFolders, DrawMode=OwnerDrawFixed) in the single live
ItemViewer variant. Its hierarchy is synthesized by FolderHierarchyBuilder.Build, which
splits the same set of at most five suggestion paths on \; it does not query the real Outlook
subfolder structure. The prediction percentage is not reliably fully visible.

This feature replaces that control with a WebView2-hosted HTML/CSS/JS breadcrumb control,
following the pattern QuickFiler already uses for its WebView2 message-body pane (feature 326
inline-image cid fix). It is the QuickFiler-surface counterpart to the EfcViewer breadcrumb
(9102) and consumes the shared live folder-hierarchy provider (9101).

Proposed Behavior

Replace the QuickFiler CboFolders ComboBox with a WebView2-hosted breadcrumb control whose
required behavior mirrors the EfcViewer surface:

1. Single-line breadcrumb Folder -> SubFolder -> Leaf, anchored at the selected/predicted leaf.
2. A leaf-only expand affordance (plus when collapsed, minus when expanded), shown only when the
   leaf has subfolders.
3. Double-clicking a non-leaf segment collapses the row after that segment, leaving a plus to the
   left of the now-terminal segment that re-expands the full breadcrumb.
4. Expanding a segment lists every real immediate Outlook subfolder of that folder via the shared
   provider from feature 9101 (live Outlook query behind an injectable seam), replacing
   FolderHierarchyBuilder.Build.
5. The prediction percentage is always fully visible and unobstructed. Capture a runtime
   reproduction of the obscuring defect first, then apply a CSS-based fix.

Build the JS<->.NET event bridge for double-click and keyboard (left/right arrow) interaction and
route the live subfolder query across that bridge.

Acceptance Criteria

• The live QuickFiler viewer variant's CboFolders ComboBox is replaced by a WebView2-hosted
  HTML/CSS/JS breadcrumb control, following the QuickFiler WebView2 message-body pane pattern.
• Each suggestion renders as a single-line breadcrumb Folder -> SubFolder -> Leaf anchored at
  the leaf.
• The leaf carries a plus/minus expand affordance only when it has subfolders.
• Double-clicking a non-leaf segment collapses the row after it and shows a plus to re-expand.
• Expanding a segment lists every real immediate Outlook subfolder via the 9101 provider, not
  only subfolders present among the top-ranked suggestions.
• The prediction percentage is fully visible and unobstructed (runtime reproduction captured,
  CSS-based fix applied).
• A JS<->.NET event bridge handles double-click and keyboard (left/right arrow) interaction and
  routes the live subfolder query.
• No third-party WinForms tree/list control and no WPF/ElementHost are introduced.
• The scoring/ranking algorithm and model output are unchanged.
• Full C# toolchain (csharpier, .NET analyzers, nullable, MSTest) is green; changed and new
  code meets repository coverage thresholds; breadcrumb core logic is unit-testable without a live
  Outlook process.

Constraints & Risks

• Do NOT introduce a third-party WinForms tree/list control or WPF/ElementHost. The control
  technology is WebView2 (HTML/CSS/JS).
• Do not change the scoring/ranking algorithm or model output; the surfaced percentage is the
  score already computed for internal ranking (feature 324 plumbing reused as-is).
• Re-verify which QuickFiler viewer variant(s) are actually live before fixing scope. The prior
  epic scoped to the single live ItemViewer and left nine other declared variants
  (QfcItemViewerV1, QfcItemViewerExpanded, etc.) untouched. Re-confirm this liveness
  assessment during research rather than assuming it still holds; record the scope decision in the
  spec.
• Live Outlook I/O must be isolated behind an injectable seam so the breadcrumb core logic is
  unit-testable without a live Outlook process.

Test Conditions

• Unit coverage: pure breadcrumb-model logic (ancestor-chain rendering, collapse/expand state
  transitions) without a live Outlook process.
• Integration scenarios: JS<->.NET bridge message routing for double-click, keyboard, and live
  subfolder query.
• Runtime reproduction of the percentage-obscuring defect, then verification the CSS fix
  resolves it.

Source

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