Match chat document search on file names and clear dropdown divider artifacts

[Paul Lizer (paullizer)]

Fixes #1256

What was broken

Two defects in the chat page grounded-search document picker.

File names were unsearchable. buildDocumentDescriptor() built the searchable text from the display name (title || file_name) plus the section label, so the file name never reached the matcher for any document that had extracted title metadata. Searching 200 for Quarterly_Report_200_final.pdf returned "No Matching Documents".

Filtering left orphaned separator lines. updateDropdownStructure() resolved divider visibility by scanning for the nearest visible sibling, but isVisibleItem() reported dividers as not visible, so the scan walked straight through them — and the always-visible data-search-role="action" row counted as valid content. Every orphaned divider found the same action row above it and the first surviving header below it, so all of them stayed visible.

before (filter "beta")      after
  - Select All                - Select All
  ------------------          # [Public] Beta
  ------------------          - Beta Handbook
  # [Public] Beta
  - Beta Handbook

What changed

chat-searchable-select.js

• New shared normalizeSearchText() / matchesSearchTokens(): text and query are lowercased with _, -, . treated as word breaks, and a row matches when every typed token appears somewhere. Matching stays position-independent, so 200 still matches mid-file-name and report 200 now matches Quarterly_Report_200_final.pdf.
• Divider visibility is now section-aware. A bound divider (one that introduces a .dropdown-header) follows that header and requires visible section content before it; an unbound divider (the static rule under "All" / "Clear All") requires visible content before and visible section content after. Always-visible action rows never count as section content. A final collapseRedundantDividers() pass guarantees no leading, trailing, or stacked separator lines regardless of future markup.

chat-documents.js / chat-onload.js / chats.css

• The file name is added to the search label and rendered as a smaller muted line under the title when the two differ, via textContent; both lines go into the row tooltip.
• The dropdown button label now reads .chat-document-option-title instead of the first descendant span, which would otherwise concatenate title and file name.

No backend change was needed — /api/documents, /api/group_documents, and /api/public_workspace_documents already return file_name, and the backend workspace list already matched on title or file name. No new browser assets, so the local-only asset policy and CSP are unaffected.

Blast radius

• The Compare modal document picker inherits both fixes, since chat-messages.js relocates the same #document-dropdown field into #document-comparison-picker-controls.
• The shared token matcher is also used by createSearchableSingleSelect, so the prompt, model, and agent selectors gain multi-word matching. This is intentional for consistency.
• Trade-off: normalizing . means a query like a.b matches text containing a and b separately. Acceptable for a filename-oriented picker, and it is what makes report 200 work.

Validation

functional_tests/test_chat_document_search_filename_matching.py — 8/8 pass. The divider and matching checks are executable, not static: the test loads the production chat-searchable-select.js into a minimal DOM shim through Node and asserts the exact filtered output for the document, scope, and tags dropdowns, including that no leading, trailing, or adjacent separator lines survive. Skips the executable section with a warning when Node.js is unavailable.

ui_tests/test_chat_document_search_filename_and_dividers.py — 4/4 pass. Mounts the real modules with production CSS, stubs fetch with personal/group/public documents, and drives loadAllDocs(). Covers the muted file-name line (stacking, smaller font, tooltip, suppressed when it duplicates the title), file-name/multi-word/literal/title matching, no orphaned separators plus structure restored on clear, and the mobile drawer without horizontal overflow. All four fail against the pre-fix source (file-name search returns No Matching Documents), so this is genuine regression coverage.

Regression suite — test_chat_search_panel_document_row_layout.py, test_chat_document_dropdown_viewport_fit.py, test_chat_scope_selector_sync.py, test_chat_document_action_selector_labels.py, test_chat_grouped_selectors.py, test_chat_compare_modal_document_picker.py all pass (skips require SIMPLECHAT_UI_BASE_URL).

Pre-existing failures, unchanged by this PR (verified by running them against a pristine HEAD): functional_tests/test_chat_searchable_selectors.py 7/11, functional_tests/test_workspace_scope_prompts_fix.py 1/2, and ui_tests/test_workflow_cancellation_controls.py when run in a batch (a Playwright sync/asyncio isolation issue — it passes alone both with and without these changes).

Version bumped to 0.250.209, with fix documentation in docs/explanation/fixes/CHAT_DOCUMENT_SEARCH_FILENAME_AND_DIVIDER_FIX.md and release notes updated.