And implemented uiAttributeBackground on OS X. Not sure what else to add besides the feature variants...

common/drawtext.c

@@ -68,13 +68,14 @@ void drawTextBackground(uiDrawContext *c, double x, double y, uiDrawTextLayout *
 		double startx, endx;
 		uiDrawPath *path;
 
+		uiDrawTextLayoutLineByteRange(layout, line, &lstart, &lend);
 		if (lend > end)		// don't cross lines
 			lend = end;
-		startx = uiDrawTextLayoutByteLocationInLine(layout, line, start);
+		startx = uiDrawTextLayoutByteLocationInLine(layout, start, line);
 		// TODO explain this; note the use of start with lend
 		endx = layoutwid;
 		if (!isSelection || end <= lend)
-			endx = uiDrawTextLayoutByteLocationInLine(layout, line, lend);
+			endx = uiDrawTextLayoutByteLocationInLine(layout, lend, line);
 		uiDrawTextLayoutLineGetMetrics(layout, line, &m);
 		path = uiDrawNewPath(uiDrawFillModeWinding);
 		uiDrawPathAddRectangle(path,

darwin/attrstr.m

@@ -9,6 +9,7 @@ struct foreachParams {
 	CFMutableAttributedStringRef mas;
 	NSMutableDictionary *converted;
 	uiDrawFontDescriptor *defaultFont;
+	NSMutableArray *backgroundBlocks;
 };
 
 static void ensureFontInRange(struct foreachParams *p, size_t start, size_t end)
@@ -40,6 +41,20 @@ static void adjustFontInRange(struct foreachParams *p, size_t start, size_t end,
 	}
 }
 
+static backgroundBlock mkBackgroundBlock(size_t start, size_t end, double r, double g, double b, double a)
+{
+	return Block_copy(^(uiDrawContext *c, uiDrawTextLayout *layout, double x, double y) {
+		uiDrawBrush brush;
+
+		brush.Type = uiDrawBrushTypeSolid;
+		brush.R = r;
+		brush.G = g;
+		brush.B = b;
+		brush.A = a;
+		drawTextBackground(c, x, y, layout, start, end, &brush, 0);
+	});
+}
+
 static int processAttribute(uiAttributedString *s, uiAttributeSpec *spec, size_t start, size_t end, void *data)
 {
 	struct foreachParams *p = (struct foreachParams *) data;
@@ -47,7 +62,11 @@ static int processAttribute(uiAttributedString *s, uiAttributeSpec *spec, size_t
 	CGColorSpaceRef colorspace;
 	CGColorRef color;
 	CGFloat components[4];
+	size_t ostart, oend;
+	backgroundBlock block;
 
+	ostart = start;
+	oend = end;
 	start = attrstrUTF8ToUTF16(s, start);
 	end = attrstrUTF8ToUTF16(s, end);
 	range.location = start;
@@ -97,6 +116,12 @@ static int processAttribute(uiAttributedString *s, uiAttributeSpec *spec, size_t
 		CFAttributedStringSetAttribute(p->mas, range, kCTForegroundColorAttributeName, color);
 		CFRelease(color);
 		break;
+	case uiAttributeBackground:
+		block = mkBackgroundBlock(ostart, oend,
+			spec->R, spec->G, spec->B, spec->A);
+		[p->backgroundBlocks addObject:block];
+		Block_release(block);
+		break;
 	// TODO
 	}
 	return 0;
@@ -154,7 +179,7 @@ static CTParagraphStyleRef mkParagraphStyle(uiDrawTextLayoutParams *p)
 	return ps;
 }
 
-CFAttributedStringRef attrstrToCoreFoundation(uiDrawTextLayoutParams *p)
+CFAttributedStringRef attrstrToCoreFoundation(uiDrawTextLayoutParams *p, NSArray **backgroundBlocks)
 {
 	CFStringRef cfstr;
 	CFMutableDictionaryRef defaultAttrs;
@@ -194,10 +219,12 @@ CFAttributedStringRef attrstrToCoreFoundation(uiDrawTextLayoutParams *p)
 	fep.mas = mas;
 	fep.converted = [NSMutableDictionary new];
 	fep.defaultFont = p->DefaultFont;
+	fep.backgroundBlocks = [NSMutableArray new];
 	uiAttributedStringForEachAttribute(p->String, processAttribute, &fep);
 	applyAndFreeFontAttributes(&fep);
 	[fep.converted release];
 	CFAttributedStringEndEditing(mas);
 
+	*backgroundBlocks = fep.backgroundBlocks;
 	return mas;
 }

darwin/drawtext.m

@@ -28,6 +28,8 @@ struct uiDrawTextLayout {
 	// we compute this once when first creating the layout
 	uiDrawTextLayoutLineMetrics *lineMetrics;
 
+	NSArray *backgroundBlocks;
+
 	// for converting CFAttributedString indices from/to byte offsets
 	size_t *u8tou16;
 	size_t nUTF8;
@@ -114,7 +116,7 @@ uiDrawTextLayout *uiDrawNewTextLayout(uiDrawTextLayoutParams *p)
 	CGRect rect;
 
 	tl = uiNew(uiDrawTextLayout);
-	tl->attrstr = attrstrToCoreFoundation(p);
+	tl->attrstr = attrstrToCoreFoundation(p, &(tl->backgroundBlocks));
 	range.location = 0;
 	range.length = CFAttributedStringGetLength(tl->attrstr);
 	tl->width = p->Width;
@@ -165,6 +167,7 @@ void uiDrawFreeTextLayout(uiDrawTextLayout *tl)
 {
 	uiFree(tl->u16tou8);
 	uiFree(tl->u8tou16);
+	[tl->backgroundBlocks release];
 	uiFree(tl->lineMetrics);
 	// TODO release tl->lines?
 	CFRelease(tl->frame);
@@ -177,8 +180,13 @@ void uiDrawFreeTextLayout(uiDrawTextLayout *tl)
 // TODO document that (x,y) is the top-left corner of the *entire frame*
 void uiDrawText(uiDrawContext *c, uiDrawTextLayout *tl, double x, double y)
 {
+	backgroundBlock b;
+
 	CGContextSaveGState(c->c);
 
+	for (b in tl->backgroundBlocks)
+		b(c, tl, x, y);
+
 	// Core Text doesn't draw onto a flipped view correctly; we have to pretend it was unflipped
 	// see the iOS bits of the first example at https://developer.apple.com/library/mac/documentation/StringsTextFonts/Conceptual/CoreText_Programming/LayoutOperations/LayoutOperations.html#//apple_ref/doc/uid/TP40005533-CH12-SW1 (iOS is naturally flipped)
 	// TODO how is this affected by a non-identity CTM?

darwin/uipriv_darwin.h

@@ -145,4 +145,5 @@ extern CTFontDescriptorRef fontdescToCTFontDescriptor(uiDrawFontDescriptor *fd);
 extern void fontdescFromCTFontDescriptor(CTFontDescriptorRef ctdesc, uiDrawFontDescriptor *uidesc);
 
 // attrstr.m
-extern CFAttributedStringRef attrstrToCoreFoundation(uiDrawTextLayoutParams *p);
+typedef void (^backgroundBlock)(uiDrawContext *c, uiDrawTextLayout *layout, double x, double y);
+extern CFAttributedStringRef attrstrToCoreFoundation(uiDrawTextLayoutParams *p, NSArray **backgroundBlocks);

examples/drawtext/attributes.c

@@ -82,6 +82,7 @@ static void setupAttributedString(void)
 	uiAttributedStringAppendUnattributed(attrstr, next);
 	spec.Type = uiAttributeBackground;
 	// Direct2D "Peach Puff" (#FFDAB9)
+	// TODO choose a darker color
 	spec.R = 1.0;
 	spec.G = 0.85490196078431372;
 	spec.B = 0.7254901960784313;