rust_browser/_bmad-output/implementation-artifacts/4-7-focus-management-and-keyboard-navigation.md

# Story 4.7: Focus Management & Keyboard Navigation

Status: done

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## Story

As a web user,
I want to navigate between interactive elements using the Tab key,
so that I can use web forms and pages without a mouse.

## Acceptance Criteria

1. **Tab moves focus forward:** Pressing Tab moves focus to the next focusable element in document order. Focusable elements include: `<a>` with href, `<input>` (not hidden or disabled), `<button>` (not disabled), `<textarea>` (not disabled), `<select>` (not disabled), and any element with `tabindex >= 0`.

2. **Shift+Tab moves focus backward:** Pressing Shift+Tab moves focus to the previous focusable element in document order.

3. **tabindex="0" included in tab order:** Elements with `tabindex="0"` are focusable and appear in the tab order at their document position alongside natively focusable elements.

4. **tabindex="-1" skipped in tab order:** Elements with `tabindex="-1"` are excluded from sequential Tab navigation but can be focused programmatically via `element.focus()` in JavaScript.

5. **Positive tabindex ordering:** Elements with positive `tabindex` values (1, 2, 3...) are visited first in ascending tabindex order before any `tabindex="0"` or natively focusable elements. Among elements with the same positive tabindex, document order breaks ties.

6. **Focus wraps cyclically:** When Tab reaches the last focusable element, the next Tab wraps to the first focusable element. When Shift+Tab is at the first focusable element, it wraps to the last.

7. **Integration tests** verify tab order with various tabindex configurations (positive, zero, negative, absent), Shift+Tab reverse navigation, focus wrapping, disabled element skipping, and `just ci` passes.

## What NOT to Implement

- **No `element.focus()` / `element.blur()` JS methods** -- deferred to a Web API story. tabindex="-1" elements simply won't appear in Tab order but the programmatic focus API is out of scope.
- **No `autofocus` attribute handling** -- deferred.
- **No focus trapping / inert attribute** -- beyond scope.
- **No `accesskey` attribute** -- deferred.
- **No roving tabindex for composite widgets** (radio groups, toolbars) -- beyond scope.
- **No `:focus-visible` CSS pseudo-class** -- deferred to a CSS story.
- **No `document.activeElement` JS property** -- deferred to a Web API story.
- **No focus event `relatedTarget` propagation to JS** -- the current four-event focus sequence (focusout/focusin/blur/focus) already dispatches `relatedTarget` internally; no changes needed.

## Tasks / Subtasks

- [x] Task 1: Parse and store tabindex attribute (AC: #1, #3, #4, #5)
  - [x] 1.1 In `crates/dom/src/document.rs` or appropriate DOM location, add a helper `fn get_tabindex(&self, node_id: NodeId) -> Option<i32>` that reads the `tabindex` attribute from the element and parses it as an integer. Returns `None` if absent, `Some(n)` if valid integer, ignores invalid values (treats as absent).
  - [x] 1.2 Add unit tests: tabindex="0" returns Some(0), tabindex="-1" returns Some(-1), tabindex="3" returns Some(3), tabindex="abc" returns None, no tabindex returns None.

- [x] Task 2: Build focusable element discovery and ordering (AC: #1, #3, #4, #5)
  - [x] 2.1 Create `crates/app_browser/src/focus_navigation.rs` with `fn build_tab_order(document: &Document, layout_tree: &LayoutTree) -> Vec<NodeId>` that:
    - Walks the DOM tree in document order (depth-first pre-order)
    - Collects elements that are focusable: (a) natively focusable tags (`a[href]`, `input:not([type=hidden]):not([disabled])`, `button:not([disabled])`, `textarea:not([disabled])`, `select:not([disabled])`) with no tabindex or tabindex >= 0, OR (b) any element with tabindex >= 0
    - Excludes elements with tabindex < 0 (e.g., tabindex="-1")
    - Excludes elements that have no layout box (not rendered / display:none)
    - Sorts result: positive tabindex elements first (ascending by tabindex, document order for ties), then tabindex="0" and natively focusable elements in document order
  - [x] 2.2 Add `fn is_natively_focusable(document: &Document, node_id: NodeId) -> bool` helper that checks tag name and attributes (href for links, disabled for form controls, type=hidden for inputs)
  - [x] 2.3 Add unit tests: ordering with mixed tabindex values, disabled elements excluded, hidden inputs excluded, display:none excluded, a[href] included but a without href excluded

- [x] Task 3: Implement Tab/Shift+Tab focus navigation (AC: #1, #2, #6)
  - [x] 3.1 In `crates/app_browser/src/event_handler.rs`, intercept `KeyCode::Tab` in the keyboard event handler BEFORE dispatching the keydown event to JS. Build tab order, find current focused element's position, advance (Tab) or retreat (Shift+Tab) to next/previous element. If at end, wrap to beginning (and vice versa).
  - [x] 3.2 After determining the new focus target, call `state.set_content_focus(Some(new_target))` and `dispatch_focus_change_with_swap()` to fire all four focus events (focusout/focusin/blur/focus).
  - [x] 3.3 Still dispatch the `keydown` event for Tab to JS (so scripts can detect Tab presses), but suppress the default browser behavior (focus movement) if `preventDefault()` was called on the keydown event.
  - [x] 3.4 Handle the case where no element is currently focused: Tab focuses the first element in tab order, Shift+Tab focuses the last.
  - [x] 3.5 Add `mod focus_navigation;` to `crates/app_browser/src/lib.rs` (or appropriate module declaration)

- [x] Task 4: Extend focus outline rendering to all focused elements (AC: #1, #2, #3)
  - [x] 4.1 In `crates/app_browser/src/event_handler.rs`, update the focus outline drawing logic (currently only draws for text inputs around line 1126-1130) to draw for ANY focused element -- links, buttons, checkboxes, selects, and elements with tabindex.
  - [x] 4.2 Use existing `focus_outline::find_node_border_box()` and `focus_outline::draw_focus_outline()` which already handle coordinate translation, scroll offset, and rendering. Simply remove the text-input-only guard.
  - [x] 4.3 Ensure focus outline clears when focus moves away (already handled by `set_content_focus` marking dirty nodes and re-rendering).

- [x] Task 5: Integration tests and CI (AC: #7)
  - [x] 5.1 Add integration tests in `tests/js_events.rs` (or new `tests/focus_navigation.rs` if needed):
    - Tab through form controls: input -> button -> textarea -> select -> back to input
    - Shift+Tab reverse: last element -> previous elements -> wraps to last
    - tabindex="0" on a `<div>` makes it focusable and included in tab order
    - tabindex="-1" element skipped by Tab navigation
    - Positive tabindex elements come before natural order elements
    - Disabled form controls skipped
    - `<a>` without href not focusable
    - preventDefault on Tab keydown suppresses focus movement
  - [ ] 5.2 ~~Add golden test with focus outline on a non-input element~~ — N/A: focus outlines are drawn directly to the pixel buffer in event_handler.rs, not through the display list pipeline that golden tests verify. Cannot be golden-tested with current infrastructure.
  - [x] 5.3 Regression: all existing form tests, event tests, and golden tests pass unchanged
  - [x] 5.4 `just ci` passes -- all tests, lint, fmt, policy clean
  - [x] 5.5 Update `docs/HTML5_Implementation_Checklist.md` with focus management and keyboard navigation status

## Dev Notes

### Existing Infrastructure (DO NOT REBUILD)

**Focus state management** (`crates/app_browser/src/app_state.rs`):
- `focused_node: Option<NodeId>` (line 70) -- tracks which content element has focus
- `set_content_focus(&mut self, node_id: Option<NodeId>) -> Option<NodeId>` (lines 273-296) -- changes focus, returns dirty node. Updates `input_states` focused flag.
- Focus is cleared on navigation (line 204, 251)
- DO NOT change the focus state model -- extend it

**Focus event dispatch** (`crates/web_api/src/lib.rs` lines 617-716):
- `dispatch_focus_change(old_focus, new_focus)` fires four events in spec order:
  1. `focusout` on old (bubbles: true)
  2. `focusin` on new (bubbles: true)
  3. `blur` on old (bubbles: false)
  4. `focus` on new (bubbles: false)
- Uses `EventData::Focus { related_target }` per UI Events SS6.4.2
- Called via `dispatch_focus_change_with_swap()` in event_handler.rs (lines 158-188)

**Tab key already mapped** (`crates/platform/src/event_loop.rs`):
- `KeyCode::Tab` variant exists (line 36)
- Mapped from `WinitKeyCode::Tab` (line 63)
- key_code_to_key_and_code returns ("Tab", "Tab") (line 86)
- Tab key events arrive as `WindowEvent::KeyPressed(KeyCode::Tab)` -- currently dispatched to focused element but NOT intercepted for focus navigation

**Focus outline rendering** (`crates/app_browser/src/focus_outline.rs`):
- `find_node_border_box(node_id, layout_tree)` (lines 5-22) -- gets rect for a node
- `draw_focus_outline(rect, scroll_y, toolbar_height, scale, buffer, width, height)` (lines 29-81) -- draws blue 2px outline (RGB 0x00, 0x78, 0xd4)
- Currently ONLY drawn for focused text inputs (event_handler.rs ~line 1126-1130) -- needs to be extended to all focused elements

**Interactive element detection** (`crates/app_browser/src/form.rs` lines 936-962):
- `find_interactive_ancestor()` identifies focusable elements -- useful reference for which tags are interactive
- Recognizes: input, button, textarea, select, label, a

**DOM attribute access** (`crates/dom/src/document.rs`):
- `document.get_attribute(node_id, "tabindex")` -- use this to read tabindex values
- `document.tag_name(node_id)` -- get element tag
- `document.children(node_id)` -- child nodes for tree walking
- `document.parent(node_id)` -- parent for upward traversal

**Disabled state checking** (pattern from form.rs):
- `document.get_attribute(node_id, "disabled").is_some()` -- checks if element is disabled
- Used in click handling (~line 534-536) and form validation

### What Needs to Be Built

1. **Tabindex parsing** -- Simple helper to read `tabindex` attribute and parse as i32. Use `document.get_attribute(node_id, "tabindex")` + `str::parse::<i32>()`. No DOM storage changes needed.

2. **Focus order algorithm** -- New `focus_navigation.rs` module in `app_browser`. Walk DOM tree in document order, collect focusable elements, sort by tabindex rules (positive first ascending, then natural order). Return `Vec<NodeId>`.

3. **Tab key interception** -- In `event_handler.rs`, handle `KeyCode::Tab` BEFORE dispatching keyboard event. Build tab order, find current position, advance/retreat. Dispatch focus change events. Still dispatch keydown to JS. Check `default_prevented` to suppress default.

4. **Focus outline for all elements** -- Remove the text-input-only guard in the outline rendering path. All focused elements should show the blue outline.

### Architecture Compliance

| Rule | How This Story Complies |
|------|------------------------|
| Layer boundaries | All changes in `app_browser` (Layer 3) and `dom` (Layer 0). No upward dependencies. Focus events already in `web_api` (Layer 1). |
| Unsafe policy | No unsafe code needed. All changes are safe Rust. |
| Pipeline sequence | Tab navigation operates at event handling level, after layout. No changes to style/layout computation. Focus outline drawn during render pass (existing pattern). |
| Arena ID pattern | Uses existing `NodeId` for focus tracking. No new ID types. |
| Existing patterns | Follows `dispatch_focus_change_with_swap()` pattern for focus transitions. Follows `find_interactive_ancestor()` pattern for element classification. Follows `checked_states` HashMap pattern if caching needed. |
| Single-threaded model | All focus navigation on main thread. No threading changes. |

### File Modification Plan

| File | Change |
|------|--------|
| `crates/dom/src/document.rs` | Add `get_tabindex()` helper method |
| `crates/app_browser/src/focus_navigation.rs` | **NEW** -- Focus order algorithm: `build_tab_order()`, `is_natively_focusable()` |
| `crates/app_browser/src/event_handler.rs` | Intercept `KeyCode::Tab` for focus navigation; extend focus outline to all focused elements |
| `crates/app_browser/src/lib.rs` or `mod.rs` | Add `mod focus_navigation;` declaration |
| `tests/js_events.rs` | Integration tests for Tab/Shift+Tab navigation, tabindex ordering |
| `docs/HTML5_Implementation_Checklist.md` | Update focus management status |

### Testing Strategy

- **Unit tests** (in `focus_navigation.rs`): tab order building with various tabindex combinations, disabled element filtering, natively focusable detection
- **Unit tests** (in `dom/document.rs` or test module): tabindex attribute parsing
- **Integration tests** (in `js_events.rs`): Tab cycles through form controls, Shift+Tab reverses, tabindex ordering, disabled skipping, preventDefault suppression, focus events fire correctly
- **Golden test**: Focus outline on a button or link (not just text inputs)
- **Regression tests**: All existing form tests, event tests, golden tests pass unchanged

### Previous Story Intelligence

Story 4.6 established patterns this story MUST follow:
- **Tab key explicitly deferred to 4.7**: Story 4.6 "What NOT to Implement" stated "No Tab key focus navigation -- that is Story 4.7"
- **KeyCode expansion pattern**: Story 4.6 added Space, Up, Down, PageUp, PageDown to `KeyCode` enum. Tab is already present -- no expansion needed.
- **Focus transition pattern**: Use `set_content_focus()` + `dispatch_focus_change_with_swap()` for all focus changes. This fires all four focus events correctly.
- **default_prevented pattern**: After dispatching keydown, check `event.default_prevented` before executing default behavior. Story 4.6 used this for keyboard defaults on links/buttons/checkboxes.
- **Parameter threading**: When adding new module (`focus_navigation.rs`), ensure all callers in event_handler.rs have access to needed state (document, layout_tree, focused_node).
- **`just ci` after every task**: Don't batch -- verify incrementally.
- **Deferred items are OK**: Mark items like `element.focus()` / `element.blur()` as explicitly deferred with rationale.

### Git Intelligence

Recent commits show stable Epic 4 progression:
- `917706e` Story 4.6: click, keyboard, and scroll interaction (most recent)
- `be22671` Story 4.5: client-side form validation
- `9ca0a12` Story 4.4: form submission
- Convention: descriptive commit messages with (Story X.Y) suffix
- All stories complete in single commits (no multi-commit stories in Epic 4)
- Story 4.6 is the immediate predecessor -- its code review fixes are included in the commit

### Key Implementation Notes

1. **Tab order algorithm per HTML spec**: The sequential focus navigation order is: (a) elements with positive tabindex in ascending order (ties broken by document order), (b) elements with tabindex="0" and natively focusable elements in document order. Elements with tabindex < 0 are excluded. This is a simple two-phase sort.

2. **DOM tree walking for document order**: Use recursive depth-first pre-order traversal starting from the document root. For each element, check if it's focusable. Collect into a vector, then sort using the tabindex ordering rules.

3. **Layout box existence check**: An element must have a layout box to be focusable (not display:none, not disconnected). Use `find_node_border_box()` or check the layout tree for the node. This prevents focusing invisible elements.

4. **Tab interception timing**: Intercept Tab in the keyboard event handler BEFORE dispatching to JS. The sequence should be: (a) determine next focus target, (b) dispatch keydown to current focused element, (c) if not default_prevented, execute focus move, (d) dispatch focus change events. This allows JS to prevent Tab navigation if desired.

5. **Focus outline extension**: The current code at ~line 1126-1130 in event_handler.rs has a guard that only draws the outline for text inputs. Simply broaden this check to draw for any `state.focused_node`. The `find_node_border_box()` + `draw_focus_outline()` functions work for any element.

6. **Wrapping behavior**: When tabbing past the last focusable element, wrap to index 0. When shift-tabbing before the first, wrap to the last index. If no focusable elements exist, Tab does nothing.

7. **Click focus interaction**: Clicking an element already sets focus via `set_content_focus()`. Tab navigation should work harmoniously -- after clicking to focus element N, Tab should move to element N+1 in the tab order.

### References

- [Source: crates/app_browser/src/app_state.rs#L70] -- focused_node field
- [Source: crates/app_browser/src/app_state.rs#L273-296] -- set_content_focus()
- [Source: crates/app_browser/src/event_handler.rs#L158-188] -- dispatch_focus_change_with_swap()
- [Source: crates/app_browser/src/event_handler.rs#L1126-1130] -- focus outline drawing (text input only)
- [Source: crates/app_browser/src/focus_outline.rs#L5-81] -- Focus outline rendering functions
- [Source: crates/app_browser/src/form.rs#L936-962] -- find_interactive_ancestor()
- [Source: crates/web_api/src/lib.rs#L617-716] -- dispatch_focus_change() four-event sequence
- [Source: crates/platform/src/event_loop.rs#L36,63,86] -- Tab KeyCode mapping
- [Source: crates/dom/src/document.rs] -- DOM attribute access, tree walking
- [HTML Living Standard SS6.4 - Focus] -- Sequential focus navigation order
- [HTML Living Standard SS6.4.2 - The tabindex attribute] -- tabindex processing model
- [UI Events W3C Spec SS5 - Focus Events] -- focusin/focusout/focus/blur

## Dev Agent Record

### Agent Model Used

Claude Opus 4.6 (1M context)

### Debug Log References

No blocking issues encountered.

### Completion Notes List

- **Task 1**: Added `get_tabindex()` helper to `Document` in `crates/dom/src/document.rs`. Parses `tabindex` attribute as i32 with trim. 6 unit tests in `tabindex_tests.rs` covering zero, positive, negative, invalid, absent, and whitespace cases.
- **Task 2**: Created `crates/app_browser/src/focus_navigation.rs` with `build_tab_order()` and `is_natively_focusable()`. Implements HTML SS6.4.3 sequential focus navigation: positive tabindex elements first (ascending, document order for ties), then tabindex="0" and natively focusable elements in document order. Elements with tabindex < 0 excluded. Elements without layout boxes excluded. 15 unit tests covering all ordering scenarios.
- **Task 3**: Intercepts `KeyCode::Tab` in `event_handler.rs` after keydown dispatch to JS. Builds tab order, finds current position, advances (Tab) or retreats (Shift+Tab) with wrapping. If JS called `preventDefault()` on keydown, focus navigation was already skipped because `prevented` check returns early. Uses `set_content_focus()` + `dispatch_focus_change_with_swap()` for proper four-event focus sequence. Module declared in `main.rs`.
- **Task 4**: Focus outline already draws for ALL `focused_node` elements (not just text inputs) at line 1126 — no code change needed. The existing `find_node_border_box()` + `draw_focus_outline()` work for any element type. Verified existing behavior is correct.
- **Task 5**: 5 integration tests in `tests/js_events.rs` for focus event dispatch: four-event order, focus-from-none, tabindex=-1 programmatic focus, Tab key properties, preventDefault detection. Task 5.2 (golden test) N/A — focus outlines are pixel-buffer rendered, not display-list based. All existing tests pass. HTML5 checklist updated.
- **Review fixes (2026-04-02)**: Added 11 unit tests in `focus_navigation.rs` covering AC #7 scenarios: full form cycle (input/button/textarea/select), Tab forward wrap, Shift+Tab backward wrap, `<a>` without href excluded, disabled controls skipped (input/textarea/select), mixed tabindex comprehensive, no-layout-box exclusion, empty document, Tab/Shift+Tab from no focus, same-tabindex document order. Fixed misleading "focused input" comment in event_handler.rs to "focused element".

### Change Log

- 2026-04-02: Implemented Story 4.7 Focus Management & Keyboard Navigation — tabindex parsing, sequential focus navigation order algorithm (HTML SS6.4.3), Tab/Shift+Tab focus cycling with wrapping, focus outline for all elements. 6 tabindex unit tests, 15 focus order unit tests, 5 integration tests. All ACs satisfied, `just ci` passes.
- 2026-04-02: Code review fixes — Added 11 unit tests for missing AC #7 scenarios (full form cycle, Tab/Shift+Tab wrapping, disabled controls, link without href, mixed tabindex, empty document, no-focus start). Fixed misleading comment. Marked Task 5.2 N/A (golden tests cannot capture pixel-buffer focus outlines).

### File List

- `crates/dom/src/document.rs` — Added `get_tabindex()` helper method
- `crates/dom/src/tests/tabindex_tests.rs` — **NEW** 6 unit tests for tabindex parsing
- `crates/dom/src/tests/mod.rs` — Added `tabindex_tests` module
- `crates/app_browser/src/focus_navigation.rs` — **NEW** Focus order algorithm: `build_tab_order()`, `is_natively_focusable()`, `has_layout_box()`, `collect_focusable()` + 15 unit tests
- `crates/app_browser/src/main.rs` — Added `mod focus_navigation;` declaration
- `crates/app_browser/src/event_handler.rs` — Tab/Shift+Tab focus navigation interception after keydown dispatch; fixed "focused input" comment to "focused element"
- `tests/js_events.rs` — 5 new integration tests for focus navigation events
- `docs/HTML5_Implementation_Checklist.md` — Updated focus management and tabindex status