Use separate pixmaps for each edge of a container (i3#3479)

include/data.h

@@ -660,7 +660,7 @@ struct Con {
 
     /* The surface used for the frame window. */
     surface_t frame;
-    surface_t frame_buffer;
+    sparse_surface_t frame_buffer;
     bool pixmap_recreated;
 
     enum {

include/libi3.h

@@ -582,15 +582,64 @@ typedef struct surface_t {
     /* The cairo object representing the drawable. In general,
      * this is what one should use for any drawing operation. */
     cairo_t *cr;
+
+    /* Indicates if the surface is initialized */
+    bool initialized;
 } surface_t;
 
+/* Enum used to indicate the top, bottom, left, or right side of a
+ * sparse surface */
+typedef enum {
+    SPARSE_SURFACE_TOP = 0,
+    SPARSE_SURFACE_BOTTOM = 1,
+    SPARSE_SURFACE_LEFT = 2,
+    SPARSE_SURFACE_RIGHT = 3,
+    SPARSE_SURFACE_SIDE_COUNT = 4
+} sparse_surface_side;
+
+/* A wrapper grouping an XCB drawable and both a graphics context
+ * and the corresponding cairo objects representing it. This is
+ * similar to a surface_t, except that it contains four drawable
+ * rectangles, one for each side of the window which allows us to
+ * create a 'sparse' pixmap, and save memory by not allocating space
+ * for the window contents that we don't draw to. */
+typedef struct sparse_surface_t {
+    /* The drawables which are being represented. */
+    surface_t side_surface[SPARSE_SURFACE_SIDE_COUNT];
+
+    /* x and y offset of each drawable rectangle */
+    int x_offset[SPARSE_SURFACE_SIDE_COUNT];
+    int y_offset[SPARSE_SURFACE_SIDE_COUNT];
+
+    /* Width and height of the entire sparse surface group */
+    int width;
+    int height;
+} sparse_surface_t;
+
+/**
+ * Checks if sparse surface has been initialized (eg, if any of the
+ * sparse surface sides are initialized).
+ *
+ */
+bool sparse_surface_initialized(sparse_surface_t *sparse_surface);
+
 /**
  * Initialize the surface to represent the given drawable.
  *
  */
 void draw_util_surface_init(xcb_connection_t *conn, surface_t *surface, xcb_drawable_t drawable,
                             xcb_visualtype_t *visual, int width, int height);
 
+/**
+ * Initialize the sparse surface to represent the given drawables.
+ *
+ */
+void draw_util_sparse_surface_create_and_init(xcb_connection_t *conn, sparse_surface_t *sparse_surface,
+                                              xcb_drawable_t frame_id,
+                                              uint16_t win_depth,
+                                              xcb_visualtype_t *visual, int width, int height,
+                                              int top_height, int bottom_height, int left_width, int right_width);
+
 /**
  * Resize the surface to the given size.
  *
@@ -603,6 +652,12 @@ void draw_util_surface_set_size(surface_t *surface, int width, int height);
  */
 void draw_util_surface_free(xcb_connection_t *conn, surface_t *surface);
 
+/**
+ * Destroys the sparse surface.
+ *
+ */
+void draw_util_sparse_surface_free(xcb_connection_t *conn, sparse_surface_t *sparse_surface, bool clear_id);
+
 /**
  * Parses the given color in hex format to an internal color representation.
  * Note that the input must begin with a hash sign, e.g., "#3fbc59".
@@ -637,13 +692,26 @@ void draw_util_rectangle(surface_t *surface, color_t color, double x, double y,
  */
 void draw_util_clear_surface(surface_t *surface, color_t color);
 
+/**
+ * Clears a sparse surface with the given color.
+ *
+ */
+void draw_util_sparse_clear_surface(sparse_surface_t *sparse_surface, color_t color);
+
 /**
  * Copies a surface onto another surface.
  *
  */
 void draw_util_copy_surface(surface_t *src, surface_t *dest, double src_x, double src_y,
                             double dest_x, double dest_y, double width, double height);
 
+/**
+ * Copies a sparse surface onto another non-sparse surface.
+ *
+ */
+void draw_util_sparse_copy_surface(sparse_surface_t *src_sparse, surface_t *dest, double src_x, double src_y,
+                                   double dest_x, double dest_y, double width, double height);
+
 /**
  * Puts the given socket file descriptor into non-blocking mode or dies if
  * setting O_NONBLOCK failed. Non-blocking sockets are a good idea for our