# lib-core.panel

Generic overlay-panel framework. Manages a single active widget at a time,
handles input dispatch (mouse click + wheel, edge-detected), renders a
titled panel overlay, and supports a context-menu layer. Designed as the
glue layer between game modules and the engine render/input surfaces.

**Version:** 0.1.1
**Lib-ID:** lib-core.panel
**Requires:** engine.render.*, engine.input.*, lib-core.input (lazy, for default-trigger)
**Tags:** panel, overlay, ui, input, context-menu

## Topology

<!-- topology:start (auto-generated; do not edit) -->
```mermaid
graph LR
  this["lib-core.panel"]
  engine_render["engine.render.*"]
  engine_input["engine.input.*"]
  lib_core_input["lib-core.input"]
  this --> engine_render
  this --> engine_input
  this -.->|lazy| lib_core_input
```
<!-- topology:end -->

## Scope (v0.1.0)

v0.1 ships a minimal single-active-widget panel system:

- Widget registry (register / unregister / open / close / toggle)
- Theme system (13 configurable keys, capability-by-declaration guard)
- Per-frame update with edge-detected mouse click + wheel dispatch
- Context-menu (show, auto-reposition to screen bounds, hit-test, auto-close)
- Default-trigger key binding via lib-core.input (lazy-required)
- pause_on_open flag for game-pause gating

**Intentional non-goals (deferred):**
- Multi-widget z-order / stacking panels
- Always-on HUD widgets (non-modal overlays)
- Keyboard navigation within widgets
- Panel animation (fade in / out)
- Screen-size from engine (no Lua-accessible get_screen_size; v0.1 falls back to 1280x720 constants matching default Sporel window config)

## API

### `panel.register(widget_id, widget_def)`

**Syntax:** `panel.register(widget_id: string, widget_def: table) -> void`

**Example:**
```lua
panel.register("inventory", {
    title        = "Inventory",
    pause_on_open = true,
    render       = function(ctx)
        -- ctx.bounds = {x, y, w, h}; ctx.theme; ctx.is_focused
        engine.render.draw_text("(empty)", ctx.bounds.x, ctx.bounds.y,
                                ctx.theme.font_size_body, ctx.theme.text_color)
    end,
    handle_input = function(ctx, event)
        -- event.kind = "click"|"wheel"
        -- event.x, event.y, event.button (click only)
        -- event.dy (wheel only)
    end,
})
```

Registers a widget in the panel registry. `widget_def.render` and
`widget_def.handle_input` must be functions; `widget_def.title` must be a
string. Loud-error on duplicate `widget_id` or missing required fields.

---

### `panel.unregister(widget_id)`

**Syntax:** `panel.unregister(widget_id: string) -> void`

Removes the widget from the registry. If the widget is currently active,
closes the panel (sets active to nil, clears any open context-menu).

---

### `panel.open(widget_id)`

**Syntax:** `panel.open(widget_id: string) -> void`

Sets `widget_id` as the active widget. Loud-error if `widget_id` has not
been registered.

---

### `panel.close()`

**Syntax:** `panel.close() -> void`

Closes the active widget and clears any open context-menu.

---

### `panel.toggle(widget_id)`

**Syntax:** `panel.toggle(widget_id: string) -> void`

If `widget_id` is currently active, closes it. Otherwise opens it. Handy
for key-binding toggle semantics without manual state tracking.

---

### `panel.is_open()`

**Syntax:** `panel.is_open() -> bool`

Returns `true` if any widget is currently active.

---

### `panel.is_pausing()`

**Syntax:** `panel.is_pausing() -> bool`

Returns `true` if the active widget has `pause_on_open = true`. Modules
can use this to gate their update loop (skip physics/AI while panel is
open).

---

### `panel.set_theme(overrides)`

**Syntax:** `panel.set_theme(overrides: table) -> void`

**Example:**
```lua
panel.set_theme({
    bg_color   = 0x101010F8,
    padding    = 12,
    font_size_body = 16,
})
```

Merges `overrides` into the active theme. Only keys declared in
`DEFAULT_THEME` are accepted (capability-by-declaration). Loud-error on
any unknown key with message `"panel.set_theme: unknown theme key '<k>'"`.

---

### `panel.get_theme()`

**Syntax:** `panel.get_theme() -> table`

Returns a shallow copy of the current merged theme. Safe to store; does
not alias internal state.

---

### `panel.bind_default_trigger(key, widget_id)`

**Syntax:** `panel.bind_default_trigger(key: string|nil, widget_id: string) -> void`

**Example:**
```lua
panel.register("inventory", { ... })
panel.bind_default_trigger("tab", "inventory")
-- or: panel.bind_default_trigger(nil, "inventory")  -- defaults to "tab"
```

Binds a keyboard key as the default toggle trigger for `widget_id`. `key`
defaults to `"tab"` when `nil`. Lazy-requires `lib-core.input` to avoid
module-load-time cycles. Loud-error if `widget_id` is not registered.

---

### `panel.update(dt)`

**Syntax:** `panel.update(dt: number) -> void`

Must be called each game-update frame. Checks the default trigger key,
reads mouse state, emits edge-detected click events (left and right
independently), and dispatches wheel events. No-ops if no widget is active
(but still tracks mouse state to avoid spurious edges on next open).

---

### `panel.render()`

**Syntax:** `panel.render() -> void`

Must be called each render frame (inside the engine render phase). Draws
the panel background, border, and title, then invokes `widget_def.render(ctx)`.
If a context-menu is open, renders it on top. No-op if no widget is active.

---

### `panel.show_context_menu(x, y, actions)`

**Syntax:** `panel.show_context_menu(x: number, y: number, actions: table) -> void`

**Example:**
```lua
panel.show_context_menu(event.x, event.y, {
    { label = "Use",  callback = function(info) use_item()  end },
    { label = "Drop", callback = function(info) drop_item() end },
})
```

Pops up a context-menu at `(x, y)`. `actions` must be a non-empty array of
`{label: string, callback: function}` tables. Auto-repositions to remain
within the screen boundary. The callback receives `{close_menu: function}`
(which is a no-op in v0.1 since the menu closes itself before invoking the
callback). Click outside any row auto-closes the menu.

---

### `panel._dispatch_event_for_test(event)` — for tests only

**Syntax:** `panel._dispatch_event_for_test(event: table) -> void`

Direct passthrough to the internal `_dispatch_event` function. Allows
test-modules to simulate input events without a running game loop (since
`M.update` is not exercised headless).

## Theme Schema

| Key | Default | Description |
|-----|---------|-------------|
| `bg_color` | `0x202020F0` | Panel background (RGBA packed) |
| `border_color` | `0x808080FF` | Panel border |
| `text_color` | `0xE0E0E0FF` | Primary text |
| `text_color_dim` | `0xA0A0A0FF` | Dimmed / secondary text |
| `selection_color` | `0x404080FF` | Row / item selection highlight |
| `context_menu_bg` | `0x303030F8` | Context-menu background |
| `context_menu_hover` | `0x404060FF` | Context-menu row hover |
| `font_size_title` | `18` | Title bar font size (px) |
| `font_size_body` | `14` | Content font size (px) |
| `padding` | `8` | Inner padding (px) |
| `row_height` | `24` | Row height for lists / menus (px) |
| `panel_width_frac` | `0.5` | Panel width as fraction of screen |
| `panel_height_frac` | `0.7` | Panel height as fraction of screen |

## Widget-Lifecycle-Contract

Widget `render(ctx)` is called each render frame while the widget is active.
Widget `handle_input(ctx, event)` is called for each dispatched input event.

Both receive a `ctx` table:

```lua
ctx = {
    bounds = {
        x = number,  -- content area top-left x
        y = number,  -- content area top-left y
        w = number,  -- content area width
        h = number,  -- content area height
    },
    theme      = table,   -- current merged theme (read-only by convention)
    is_focused = bool,    -- true when this widget is the active one
}
```

The `event` table passed to `handle_input`:

```lua
-- Mouse click:
event = { kind = "click", x = number, y = number, button = "left"|"right" }

-- Mouse wheel:
event = { kind = "wheel", dy = number }  -- dy > 0 = scroll up, dy < 0 = scroll down
```

Context-menu intercepts clicks before they reach `handle_input`. The widget
calls `panel.show_context_menu(x, y, actions)` from within `handle_input`
when a right-click (or any application-specific trigger) warrants a menu.

## Glue-Pattern

Minimal module setup (copy-paste-able):

```lua
local panel = require("lib-core.panel")

-- 1. Register your widget once (e.g. in module init or M.load)
panel.register("inventory", {
    title         = "Inventory",
    pause_on_open = true,
    render        = function(ctx) inventory_ui.draw(ctx) end,
    handle_input  = function(ctx, event) inventory_ui.on_input(ctx, event) end,
})

-- 2. Bind a toggle key (optional; defaults to Tab)
panel.bind_default_trigger("i", "inventory")

-- 3. Wire into your module's update + render
function M.update(dt)
    panel.update(dt)
    if not panel.is_pausing() then
        -- normal game logic here
    end
end

function M.render()
    -- ... draw world ...
    panel.render()  -- draws panel overlay on top
end
```