(svn r25564) -Add: a simple clipping rule to decide whether a string without linebreaks is visible before computing the layouting. This skips around 2/3 of layout computations in usual-sized list-like windows.

src/gfx.cpp

@@ -521,9 +521,22 @@ static int DrawLayoutLine(ParagraphLayout::Line *line, int y, int left, int righ
  *               will be drawn in the right direction.
  * @param underline Whether to underline what has been drawn or not.
  * @param fontsize The size of the initial characters.
+ * @return In case of left or center alignment the right most pixel we have drawn to.
+ *         In case of right alignment the left most pixel we have drawn to.
  */
 int DrawString(int left, int right, int top, const char *str, TextColour colour, StringAlignment align, bool underline, FontSize fontsize)
 {
+	/* The string may contain control chars to change the font, just use the biggest font for clipping. */
+	int max_height = max(max(FONT_HEIGHT_SMALL, FONT_HEIGHT_NORMAL), max(FONT_HEIGHT_LARGE, FONT_HEIGHT_MONO));
+
+	/* Funny glyphs may extent outside the usual bounds, so relax the clipping somewhat. */
+	int extra = max_height / 2;
+
+	if (_cur_dpi->top + _cur_dpi->height + extra < top || _cur_dpi->top > top + max_height + extra ||
+			_cur_dpi->left + _cur_dpi->width + extra < left || _cur_dpi->left > right + extra) {
+		return 0;
+	}
+
 	Layouter layout(str, INT32_MAX, colour, fontsize);
 	if (layout.Length() == 0) return 0;
 
@@ -543,6 +556,8 @@ int DrawString(int left, int right, int top, const char *str, TextColour colour,
  *               will be drawn in the right direction.
  * @param underline Whether to underline what has been drawn or not.
  * @param fontsize The size of the initial characters.
+ * @return In case of left or center alignment the right most pixel we have drawn to.
+ *         In case of right alignment the left most pixel we have drawn to.
  */
 int DrawString(int left, int right, int top, StringID str, TextColour colour, StringAlignment align, bool underline, FontSize fontsize)
 {