feat: implement Openbox-style bottom window handle and grips

Add full handle/grip assembly to the bottom of SSD window frames,
following the Openbox themerc specification for geometry and theming.
Theme parsing:
- Parse window.handle.width (handle bar height, default 6)
- Parse window.grip.width (corner grip width, default 20)
- Parse window.[active|inactive].handle.bg with Solid/Gradient support
- Parse window.[active|inactive].grip.bg (inherits from handle if unset)
- Pre-render 1px-wide fill buffers and cairo patterns for handle/grip
Scene graph (new ssd-handle.c):
- Handle assembly replaces bottom border when active, with its own
  left/right/top borders and three-segment bottom border
- Grips at left/right corners for diagonal resize (sw/se-resize)
- Center handle for vertical resize (s-resize)
- Vertical separator lines between grips and handle using border color
- Per Openbox spec, handle_width is content-only height with borders
  drawn around it (total assembly height = 2*border_width + handle_width)
Interactive visual states (grips only):
- Hover: 20% black overlay on grip content area
- Pressed: 40% black overlay with 1px inset shadow (dark top/left,
  light bottom/right) for a pushed-in 3D effect
- Dragging: 20% overlay with inset shadow maintained
- Global hover tracking (server.hovered_handle_ssd/element) ensures
  proper cleanup when cursor moves across views or to desktop
Decoration toggle cycle (ToggleDecorations action):
- New LAB_SSD_MODE_BORDER_HANDLE between BORDER and FULL
- keepBorder=true: full -> border+handle -> border -> none -> full
- keepBorder=false: full -> none -> full (unchanged)
Node types and input:
- New LAB_NODE_HANDLE, LAB_NODE_GRIP_LEFT, LAB_NODE_GRIP_RIGHT
- Integrated into LAB_NODE_BORDER/BORDER_BOTTOM containment so
  existing Border context mousebinds (Resize) work automatically
- Handle/grip descriptors resolved directly in get_cursor_context()
  bypassing ssd_get_resizing_type() for precise cursor shapes
Visibility rules:
- Hidden when maximized, shaded, or handle_width is 0
- Hidden in LAB_SSD_MODE_BORDER and LAB_SSD_MODE_NONE states
- Bottom border in ssd-border.c disabled when handle is active
Documentation:
- labwc-theme.5.scd: document all handle/grip theme properties
- labwc-actions.5.scd: update ToggleDecorations to 4-state cycle
- docs/themerc: add handle/grip default values

include/common/node-type.h

@@ -43,6 +43,10 @@ enum lab_node_type {
 	LAB_NODE_BORDER_LEFT,
 	LAB_NODE_BORDER,
 
+	LAB_NODE_HANDLE,
+	LAB_NODE_GRIP_LEFT,
+	LAB_NODE_GRIP_RIGHT,
+
 	LAB_NODE_CLIENT,
 	LAB_NODE_FRAME,
 	LAB_NODE_ROOT,

include/config/types.h

@@ -38,6 +38,7 @@ enum lab_rotation {
 enum lab_ssd_mode {
 	LAB_SSD_MODE_NONE = 0,
 	LAB_SSD_MODE_BORDER,
+	LAB_SSD_MODE_BORDER_HANDLE,
 	LAB_SSD_MODE_FULL,
 	LAB_SSD_MODE_INVALID,
 };

include/labwc.h

@@ -229,6 +229,10 @@ struct server {
 
 	struct ssd_button *hovered_button;
 
+	/* Track which handle/grip element is hovered (for cross-view cleanup) */
+	struct ssd *hovered_handle_ssd;
+	int hovered_handle_element; /* -1 = none */
+
 	/* Tree for all non-layer xdg/xwayland-shell surfaces */
 	struct wlr_scene_tree *workspace_tree;
 

include/ssd-internal.h

@@ -12,6 +12,35 @@ struct ssd_state_title_width {
 	bool truncated;
 };
 
+/* Interaction state for handle and grip elements */
+enum ssd_handle_state {
+	SSD_HANDLE_STATE_NORMAL = 0,
+	SSD_HANDLE_STATE_HOVER,
+	SSD_HANDLE_STATE_PRESSED,  /* clicked but not yet moved */
+	SSD_HANDLE_STATE_DRAGGING, /* clicked and moved */
+};
+
+/* Indices into ssd_handle_scene.element_states[] */
+#define SSD_HANDLE_ELEMENT_GRIP_LEFT  0
+#define SSD_HANDLE_ELEMENT_CENTER     1
+#define SSD_HANDLE_ELEMENT_GRIP_RIGHT 2
+#define SSD_HANDLE_ELEMENT_COUNT      3
+
+/* Indices into ssd_handle_subtree.rects[] for border/separator rects */
+enum handle_rect_idx {
+	HRECT_BORDER_LEFT,
+	HRECT_BORDER_RIGHT,
+	HRECT_BORDER_BOTTOM_LEFT,
+	HRECT_BORDER_BOTTOM_CENTER,
+	HRECT_BORDER_BOTTOM_RIGHT,
+	HRECT_BORDER_TOP_LEFT,
+	HRECT_BORDER_TOP_CENTER,
+	HRECT_BORDER_TOP_RIGHT,
+	HRECT_SEPARATOR_LEFT,
+	HRECT_SEPARATOR_RIGHT,
+	HRECT_COUNT,
+};
+
 /*
  * The scene-graph of SSD looks like below. The parentheses indicate the
  * type of each node (enum lab_node_type, stored in the node_descriptor
@@ -50,6 +79,14 @@ struct ssd_state_title_width {
  * +--extents
  *    +--top
  *    +--...
+ * +--handle (optional, when handle_width > 0)
+ *    +--inactive
+ *    |  +--textures[3] (grip_left, handle, grip_right wlr_scene_buffers)
+ *    |  +--rects[HRECT_COUNT] (borders and separators, all border_color)
+ *    |  +--overlay[3] (per-element hover/pressed dimming)
+ *    |  +--inset shadow rects (per-element top, left, bottom, right)
+ *    +--active
+ *       +--...
  */
 struct ssd {
 	struct view *view;
@@ -127,6 +164,27 @@ struct ssd {
 		} subtrees[2]; /* indexed by enum ssd_active_state */
 	} shadow;
 
+	/* Bottom handle/grip assembly (optional, created when handle_width > 0) */
+	struct ssd_handle_scene {
+		struct wlr_scene_tree *tree;
+		struct ssd_handle_subtree {
+			struct wlr_scene_tree *tree;
+			/* Per-element texture buffers (1px wide, stretched) */
+			struct wlr_scene_buffer *textures[SSD_HANDLE_ELEMENT_COUNT];
+			/* Border and separator rects (all border_color) */
+			struct wlr_scene_rect *rects[HRECT_COUNT];
+			/* Per-element overlays for hover/pressed dimming */
+			struct wlr_scene_rect *overlay[SSD_HANDLE_ELEMENT_COUNT];
+			/* Inset shadow rects (per-element, for pressed/dragging) */
+			struct wlr_scene_rect *inset_top[SSD_HANDLE_ELEMENT_COUNT];
+			struct wlr_scene_rect *inset_left[SSD_HANDLE_ELEMENT_COUNT];
+			struct wlr_scene_rect *inset_bottom[SSD_HANDLE_ELEMENT_COUNT];
+			struct wlr_scene_rect *inset_right[SSD_HANDLE_ELEMENT_COUNT];
+		} subtrees[2]; /* indexed by enum ssd_active_state */
+		/* Interaction state per element */
+		enum ssd_handle_state element_states[SSD_HANDLE_ELEMENT_COUNT];
+	} handle;
+
 	/*
 	 * Space between the extremities of the view's wlr_surface
 	 * and the max extents of the server-side decorations.
@@ -186,4 +244,11 @@ void ssd_shadow_create(struct ssd *ssd);
 void ssd_shadow_update(struct ssd *ssd);
 void ssd_shadow_destroy(struct ssd *ssd);
 
+void ssd_handle_create(struct ssd *ssd);
+void ssd_handle_update(struct ssd *ssd);
+void ssd_handle_destroy(struct ssd *ssd);
+void ssd_handle_set_element_state(struct ssd *ssd, int element,
+	enum ssd_handle_state state);
+void ssd_handle_clear_all_states(struct ssd *ssd);
+
 #endif /* LABWC_SSD_INTERNAL_H */

include/ssd.h

@@ -50,8 +50,10 @@ void ssd_set_titlebar(struct ssd *ssd, bool enabled);
 
 void ssd_enable_keybind_inhibit_indicator(struct ssd *ssd, bool enable);
 void ssd_enable_shade(struct ssd *ssd, bool enable);
+void ssd_set_handle(struct ssd *ssd, bool enabled);
 
 void ssd_update_hovered_button(struct wlr_scene_node *node);
+void ssd_update_hovered_handle(struct wlr_scene_node *node);
 
 void ssd_button_free(struct ssd_button *button);
 

include/theme.h

@@ -61,6 +61,16 @@ struct theme_background {
 struct theme {
 	int border_width;
 
+	/*
+	 * Height of the bottom handle bar (Openbox calls this
+	 * "window.handle.width" but it is the vertical size).
+	 * Set to 0 to disable the handle/grip assembly.
+	 */
+	int handle_width;
+
+	/* Width of each corner grip in the handle bar */
+	int grip_width;
+
 	/*
 	 * the space between title bar border and
 	 * buttons on the left/right/top
@@ -130,6 +140,16 @@ struct theme {
 		struct lab_data_buffer *shadow_corner_top;
 		struct lab_data_buffer *shadow_corner_bottom;
 		struct lab_data_buffer *shadow_edge;
+
+		/* handle and grip backgrounds (Openbox-compatible) */
+		struct theme_background handle_bg;
+		struct theme_background grip_bg;
+
+		/* Pre-rendered handle/grip fill buffers and patterns */
+		cairo_pattern_t *handle_pattern;
+		cairo_pattern_t *grip_pattern;
+		struct lab_data_buffer *handle_fill;
+		struct lab_data_buffer *grip_fill;
 	} window[2];
 
 	/* Derived from font sizes */

include/view.h

@@ -562,6 +562,7 @@ bool view_is_tiled_and_notify_tiled(struct view *view);
 bool view_is_floating(struct view *view);
 void view_move_to_workspace(struct view *view, struct workspace *workspace);
 bool view_titlebar_visible(struct view *view);
+bool view_handle_visible(struct view *view);
 void view_set_ssd_mode(struct view *view, enum lab_ssd_mode mode);
 void view_set_decorations(struct view *view, enum lab_ssd_mode mode, bool force_ssd);
 void view_toggle_fullscreen(struct view *view);