feat(chat): view context — tell the assistant what you're looking at

An eye next to the paperclip shares what the user has open with their next
message: the page, the folder being browsed, the file open in the viewer and
any highlighted passage (line numbers where a source view exists), plus which
entity a detail page is about. The bag is client-authored {label, value} pairs
in English — the backend only clamps (chars, never bytes), neutralizes the
harness tag and renders one <system-extra> block per message, deduped
consecutively so it appears exactly when the view changed. On by default,
per-device toggle, hover/tap to preview, a chip on every sent message;
docs/view-context.md for users, an updated harness.md clause for the model.

web/components/shared/file-viewer-base.js

@@ -5,6 +5,7 @@ import { LightElement, renderMarkdown } from '../../lib/base.js';
 import { codeLangForExt, highlightCode } from '../../lib/highlight.js';
 import { fileWatcher }    from '../../lib/file-watcher.js';
 import { t }              from '../../lib/i18n.js';
+import { setSlice, clearSlice } from '../../lib/view-context.js';
 import './pdf-view.js';   // registers <pdf-view>; pdf.js itself is imported lazily
 
 /**
@@ -119,6 +120,38 @@ function rewriteMarkdownAssets(htmlStr, baseDir, rev) {
  * source line right after) so the user can read it — or paste it straight into
  * an agent. Falls back to the log tail when no error line is recognised.
  */
+// ── View context (see `lib/view-context.js`) ─────────────────────────────────
+// Doing this in the base means doing it once: desktop and mobile inherit the
+// same behaviour, not just the same markup. One viewer is on screen per shell,
+// so a single key per slice is right.
+const VIEW_FILE_SLICE      = 'file';
+const VIEW_SELECTION_SLICE = 'selection';
+
+// How the file is being *shown*, which is not the same as what it is: the model
+// needs to know whether the user is looking at source (where a line number
+// means something) or at a rendering of it — and, for an image or a PDF, that
+// the person is seeing something the text of the message does not carry.
+const VIEW_KINDS = {
+  image:  'an image',
+  pdf:    'a PDF',
+  svg:    'an SVG image',
+  html:   'a rendered HTML page',
+  latex:  'a compiled LaTeX document',
+  binary: 'a binary file, whose content is not displayed',
+};
+
+/**
+ * 1-based first and last line covered by `[start, end)` in `text`.
+ *
+ * The end is measured one character back so a selection stopping exactly at a
+ * line break claims the line it ends on, not the empty one after it.
+ */
+function lineRange(text, start, end) {
+  const first = text.slice(0, start).split('\n').length;
+  const last  = text.slice(0, Math.max(start, end - 1)).split('\n').length;
+  return { first, last };
+}
+
 function formatLatexError(log) {
   if (!log) return '';
   const lines = log.split('\n');
@@ -186,10 +219,22 @@ export class FileViewerBase extends LightElement {
     this._watchPath   = null;     // path currently being watched (async-verified)
     this._watchUnsub  = null;     // unsubscribe function returned by fileWatcher
     this._reloadTimer = null;     // debounce timer for change-triggered reloads
+    this._selTimer    = null;     // debounce timer for selection capture
+    this._onSelectionChange = () => this._scheduleSelectionCapture();
+  }
+
+  connectedCallback() {
+    super.connectedCallback();
+    // `selectionchange` only exists on the document, so the listener is global
+    // and the filtering (is this selection inside *my* body?) is ours.
+    document.addEventListener('selectionchange', this._onSelectionChange);
   }
 
   disconnectedCallback() {
     super.disconnectedCallback();
+    document.removeEventListener('selectionchange', this._onSelectionChange);
+    if (this._selTimer) clearTimeout(this._selTimer);
+    this._clearViewContext();
     this._teardownWatch();
     if (this._reloadTimer) clearTimeout(this._reloadTimer);
     this._revokeBlobUrl();
@@ -220,6 +265,128 @@ export class FileViewerBase extends LightElement {
   _hide() {
     this._reset();
     this._teardownWatch();
+    // Both viewers stay in the DOM while hidden, so nothing else would take
+    // these down: a file the user closed is context that lies.
+    this._clearViewContext();
+  }
+
+  // ── View context ────────────────────────────────────────────────────────────
+
+  _clearViewContext() {
+    clearSlice(VIEW_FILE_SLICE);
+    clearSlice(VIEW_SELECTION_SLICE);
+  }
+
+  /**
+   * How this file is being shown, in one English noun phrase.
+   *
+   * The two view/source toggles are folded in because they change the answer:
+   * saying "rendered Markdown" while the user is in the editor would contradict
+   * the line numbers the selection slice is putting on the very same file.
+   */
+  _viewDescription() {
+    if (this._kind === 'html') {
+      return this._htmlMode === 'source' ? 'HTML source' : VIEW_KINDS.html;
+    }
+    if (this._kind === 'text') {
+      const ext = extOf(this._path);
+      if (ext !== 'md' && ext !== 'markdown') return 'source text';
+      return this._mdMode === 'edit' && this._canWrite
+        ? 'Markdown source, open in the editor'
+        : 'rendered Markdown';
+    }
+    return VIEW_KINDS[this._kind] ?? 'source text';
+  }
+
+  /**
+   * Announce the open file. Called on every non-silent load — so a new file, a
+   * reload and stepping into a past revision all refresh it — and never on the
+   * watcher's silent reload, where nothing the model would care about moved.
+   */
+  _publishFile() {
+    if (!this._path) return;
+    setSlice(VIEW_FILE_SLICE, [{
+      label: 'Open file',
+      value: `${this._path} (shown as ${this._viewDescription()})`,
+    }]);
+  }
+
+  _scheduleSelectionCapture() {
+    if (!this._path) return;
+    if (this._selTimer) clearTimeout(this._selTimer);
+    this._selTimer = setTimeout(() => this._captureSelection(), 200);
+  }
+
+  /**
+   * Keep the last non-empty selection made inside this viewer.
+   *
+   * Reading it at send time instead would be too late: by then the focus has
+   * moved to the composer's textarea and the document selection is gone. Hence
+   * also the asymmetry — a selection that collapses (the user clicked
+   * somewhere) or one made outside the viewer leaves the slice standing, and
+   * only a change of file clears it.
+   */
+  _captureSelection() {
+    this._selTimer = null;
+    if (!this._path) return;
+    const item = this._readSelection();
+    if (item) setSlice(VIEW_SELECTION_SLICE, [item]);
+  }
+
+  _readSelection() {
+    // The Markdown source editor first: a textarea owns its selection, and
+    // `window.getSelection()` says nothing about what is highlighted inside it.
+    const ta = this.querySelector('.fv-edit-textarea');
+    if (ta && document.activeElement === ta && ta.selectionStart !== ta.selectionEnd) {
+      const start = ta.selectionStart;
+      const end   = ta.selectionEnd;
+      return this._selectionItem(ta.value.slice(start, end), lineRange(ta.value, start, end));
+    }
+    const sel = window.getSelection?.();
+    if (!sel || sel.isCollapsed || sel.rangeCount === 0) return null;
+    const range = sel.getRangeAt(0);
+    // Restricted to the file's body: text selected in the chat itself — or in
+    // this page's own header — is not something the user is pointing at.
+    const body = this.querySelector('.fv-body') ?? this;
+    if (!body.contains(range.commonAncestorContainer)) return null;
+    return this._selectionItem(sel.toString(), this._selectionLines(range));
+  }
+
+  /**
+   * Line numbers, but only where a source view exists.
+   *
+   * `pre.fv-code` is rendered for plain text, for an HTML file in source mode
+   * and for a LaTeX file whose compile failed — never for rendered Markdown, a
+   * PDF or an image, where a DOM selection has no obvious mapping back to the
+   * source. There the label carries the text alone, which is enough: the model
+   * can find it with a grep.
+   */
+  _selectionLines(range) {
+    const pre = this.querySelector('pre.fv-code');
+    if (!pre || !pre.contains(range.commonAncestorContainer)) return null;
+    try {
+      const before = document.createRange();
+      before.selectNodeContents(pre);
+      before.setEnd(range.startContainer, range.startOffset);
+      const start = before.toString().length;
+      // Counted in the element's own text rather than in `_content`: syntax
+      // highlighting wraps the source in spans, and measuring both the offset
+      // and the lines against the same DOM is what keeps them in step.
+      return lineRange(pre.textContent ?? '', start, start + range.toString().length);
+    } catch {
+      return null;      // a detached or reordered range: the text alone will do
+    }
+  }
+
+  _selectionItem(text, lines) {
+    if (!text || !text.trim()) return null;
+    let label = 'Selected text';
+    if (lines) {
+      label += lines.first === lines.last
+        ? ` (line ${lines.first})`
+        : ` (lines ${lines.first}-${lines.last})`;
+    }
+    return { label, value: text };
   }
 
   /**
@@ -299,6 +466,11 @@ export class FileViewerBase extends LightElement {
       this._loading = true;
       this._versions = null; // no stale history button while the new file loads
       this._loadVersions(path);
+      // Say what is open before the bytes land — a message sent while the fetch
+      // is in flight is still a message about this file — and drop the previous
+      // file's selection, which belongs to a document nobody is looking at now.
+      this._publishFile();
+      clearSlice(VIEW_SELECTION_SLICE);
     } else {
       // Silent reload (file changed externally): keep showing the old content
       // until the new fetch lands; only update visible state on success.
@@ -496,6 +668,7 @@ export class FileViewerBase extends LightElement {
 
   _toggleHtmlMode() {
     this._htmlMode = this._htmlMode === 'preview' ? 'source' : 'preview';
+    this._publishFile();
   }
 
   // ── Markdown View | Edit ────────────────────────────────────────────────────
@@ -510,6 +683,7 @@ export class FileViewerBase extends LightElement {
       if (!this._editDirty) this._editBuffer = this._content;
     }
     this._mdMode = mode;
+    this._publishFile();
   }
 
   _onEditInput(e) {
@@ -524,6 +698,7 @@ export class FileViewerBase extends LightElement {
     this._editDirty  = false;
     this._conflict   = false;
     this._mdMode     = 'view';
+    this._publishFile();
   }
 
   /**