Move html textarea's selection colour chooser to plot_style.h, as it could be used elsewhere.

desktop/plot_style.h

@@ -64,6 +64,14 @@
 	(((((c0 >>  8) & 0xff) + ((c1 >>  8) & 0xff)) >> 1) <<  8) |	\
 	(((( c0        & 0xff) + ( c1        & 0xff)) >> 1) <<  0)
 
+/* Choose either black or white, depending on which is furthest from the
+ * percieved lightness of the supplied colour, c0. */
+#define colour_to_bw_furthest(c0)					\
+	((((((c0 & 0x0000ff) *  77) >>  8) +				\
+	   (((c0 & 0x00ff00) * 151) >> 16) +				\
+	   (((c0 & 0xff0000) *  28) >> 24)) >				\
+	  (0xff / 2)) ? 0x000000 : 0xffffff)
+
 /* get a bitmap pixel (image/bitmap.h) into a plot colour */
 #define pixel_to_colour(b)					\
 	b[0] | (b[1] << 8) | (b[2] << 16) | (b[3] << 24)

render/box_textarea.c

@@ -266,14 +266,9 @@ bool box_textarea_create_textarea(html_content *html,
 	ta_setup.text = fstyle;
 	ta_setup.text.background = NS_TRANSPARENT;
 	/* Make selected text either black or white, as gives greatest contrast
-	 * with background colour. (Calc lightness of background colour and
-	 * choose the one the lightness is furthest from.) */
-	ta_setup.selected_text =
-			(((((fstyle.foreground & 0x0000ff)      ) * 19) / 64 +
-			  (((fstyle.foreground & 0x00ff00) >>  8) * 38) / 64 +
-			  (((fstyle.foreground & 0xff0000) >> 16) *  7) / 64) >
-			 (0xff / 2)) ? 0x000000 : 0xffffff;
+	 * with background colour. */
 	ta_setup.selected_bg = fstyle.foreground;
+	ta_setup.selected_text = colour_to_bw_furthest(ta_setup.selected_bg);
 
 	/* Hand reference to dom text over to gadget */
 	gadget->data.text.initial = dom_text;