I decided that using 4 SPACES is better than mixing with tabs.

box.c

@@ -23,6 +23,13 @@ static void draw_container(struct box *, double x, double y, double width, doubl
 static void draw_button(struct box *, double x, double y, double width, double height);
 
 
+void
+append_box(struct box *parent, struct box *child)
+{
+    parent->children[parent->length++] = child;
+}
+
+
 static void
 draw_rectangle(double fx, double fy, double fwidth, double fheight)
 {

box.h

@@ -49,5 +49,6 @@ struct box *new_vertical_box(void);
 struct box *new_horizontal_box(void);
 struct box *new_chart_box(void);
 
+void append_box(struct box *parent, struct box *child);
 
 void draw_box(struct box *, double x, double y, double width, double height);

main.c

@@ -24,9 +24,8 @@ main(void)
     desc = new_button_box("A crypto-scam :-)");
 
     top = new_vertical_box();
-	top->length = 2;
-	top->children[0] = title;
-	top->children[1] = desc;
+    append_box(top, title);
+    append_box(top, desc);
 
     left = new_chart_box();
     left->fills = 5;
@@ -39,23 +38,20 @@ main(void)
     sell->color = UI_BLUE;
 
     right = new_vertical_box();
-	right->length = 3;
-	right->children[0] = buy;
-	right->children[1] = sell;
-	right->children[2] = scam;
+    append_box(right, buy);
+    append_box(right, sell);
+    append_box(right, scam);
 
     bottom = new_horizontal_box();
     bottom->fills = 3;
-	bottom->length = 2;
-	bottom->children[0] = left;
-	bottom->children[1] = right;
+    append_box(bottom, left);
+    append_box(bottom, right);
 
     ui = new_vertical_box();
-	ui->length = 2;
-	ui->children[0] = top;
-	ui->children[1] = bottom;
+    append_box(ui, top);
+    append_box(ui, bottom);
 
-	draw_box(ui, 0, 0, ui_width(), 40);
+    draw_box(ui, 0, 0, ui_width()-1, ui_height()-1);
 
     ui_refresh();
     

ui.c

@@ -11,7 +11,8 @@
 #include "ui.h"
 
 
-void ui_init(void)
+void
+ui_init(void)
 {
     srand(time(NULL));
     initscr();