# FindHighlightAdorner

The **FindHighlightAdorner** draws the Find highlights over the
[MarkdownRichEditor](MarkdownRichEditor.md)'s Visual Document: a translucent fill behind every
Match, with the Current Match filled more strongly and outlined. It is the visible half of Find; the
matching itself is the pure [`MatchFinder`](../../src/UI/Find/MatchFinder.cs).

- **Class:** `UI.Controls.FindHighlightAdorner` (derives from `System.Windows.Documents.Adorner`)
- **Attached by:** `MarkdownRichEditor` on `Loaded`, into the editor's `AdornerLayer`.

## Division of labour

Find is split so that only the editor Control touches the document:

1. **The editor computes.** `MarkdownRichEditor` builds a plain-text snapshot of the Visual Document
   (concatenating text runs, with a separator inserted at block boundaries so a Match never bridges
   two blocks), runs `MatchFinder` over it, and maps each Match back to a `TextRange`. It tracks
   which Match is the **Current Match** and exposes `MatchCount` / `MatchSummary` for the Find Bar.
2. **The adorner draws.** The editor calls `Update` with the Match ranges and the Current Match
   index; `OnRender` paints a rounded fill behind each on-screen Match, using a stronger fill and an
   outline for the Current Match. `SetColors` lets the editor pass palette-derived brushes so the
   highlights follow the light/dark theme.

## How it works

- **Repaint (cheap).** Scrolling and resizing only repaint from the ranges already held — they never
  re-run the search. Off-screen and wrapped Matches are skipped rather than painted.
- **View-only (INV-016).** The adorner is presentation-only (`IsHitTestVisible = false`) and only
  ever draws. Neither it nor the Find state on the editor feeds back into Capture, so finding,
  highlighting, and moving the Current Match never change the Markdown Document. Replacing a Match
  does edit (INV-022), but that is the editor's own `Replace` / `Replace All` — the adorner is not
  involved, and simply repaints from the fresh ranges afterwards.
- **Stale pointers.** When the document is replaced the editor recomputes the ranges; a repaint that
  races that replacement swallows the resulting `InvalidOperationException` and waits for the fresh
  ranges.