Improve frame height calculation

Instead of the position of the frame, use the position of the content
box within the frame when calculating the frame height. Adds
frame->get_content_box to retrieve the position and dimensions of the
content box.

Closes #1288

src/Frame.php

@@ -426,7 +426,7 @@ public function get_children()
     /**
      * Containing block dimensions
      *
-     * @param $i string The key of the wanted containing block's dimension (x, y, x, h)
+     * @param $i string The key of the wanted containing block's dimension (x, y, w, h)
      *
      * @return float[]|float
      */
@@ -517,6 +517,38 @@ public function get_break_margins()
         ), $this->_containing_block["h"]);
     }
 
+    /**
+     * Return the content box (x,y,w,h) of the frame
+     *
+     * @return array
+     */
+    public function get_content_box()
+    {
+        $style = $this->_style;
+        $cb = $this->_containing_block;
+
+        $x = $this->_position["x"] +
+            (float)$style->length_in_pt(array($style->margin_left,
+                    $style->border_left_width,
+                    $style->padding_left),
+                $cb["w"]);
+
+        $y = $this->_position["y"] +
+            (float)$style->length_in_pt(array($style->margin_top,
+                    $style->border_top_width,
+                    $style->padding_top),
+                $cb["h"]);
+
+        $w = $style->length_in_pt($style->width, $cb["w"]);
+
+        $h = $style->length_in_pt($style->height, $cb["h"]);
+
+        return array(0 => $x, "x" => $x,
+            1 => $y, "y" => $y,
+            2 => $w, "w" => $w,
+            3 => $h, "h" => $h);
+    }
+
     /**
      * Return the padding box (x,y,w,h) of the frame
      *

src/FrameDecorator/AbstractFrameDecorator.php

@@ -270,6 +270,14 @@ function get_margin_width()
         return $this->_frame->get_margin_width();
     }
 
+    /**
+     * @return array
+     */
+    function get_content_box()
+    {
+        return $this->_frame->get_content_box();
+    }
+
     /**
      * @return array
      */

src/FrameReflower/Block.php

@@ -218,8 +218,8 @@ protected function _calculate_content_height()
         $lines = $this->_frame->get_line_boxes();
         if (count($lines) > 0) {
             $last_line = end($lines);
-            $position = $this->_frame->get_position();
-            $height = $last_line->y + $last_line->h - $position['y'];
+            $content_box = $this->_frame->get_content_box();
+            $height = $last_line->y + $last_line->h - $content_box["y"];
         }
         return $height;
     }