text: ensure runereader behaves same as string

This commit fixes a subtle discrepancy in the handling of text input
within the shaper. Text provided as an io.RuneReader with a trailing
newline would generate an extra (empty) line of text, whereas the
same input provided as a string would not.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

text/shaper.go

@@ -223,11 +223,13 @@ func (l *Shaper) layoutText(params Parameters, minWidth, maxWidth int, lc system
 			}
 			done = endByte == len(str)
 		}
-		l.txt.append(l.layoutParagraph(params, minWidth, maxWidth, lc, str[startByte:endByte], l.paragraph))
-		if truncating {
-			params.MaxLines = maxLines - len(l.txt.lines)
-			if params.MaxLines == 0 {
-				done = true
+		if startByte != endByte || len(l.paragraph) > 0 {
+			l.txt.append(l.layoutParagraph(params, minWidth, maxWidth, lc, str[startByte:endByte], l.paragraph))
+			if truncating {
+				params.MaxLines = maxLines - len(l.txt.lines)
+				if params.MaxLines == 0 {
+					done = true
+				}
 			}
 		}
 		if done {

text/shaper_test.go

@@ -1,6 +1,7 @@
 package text
 
 import (
+	"strings"
 	"testing"
 
 	nsareg "eliasnaur.com/font/noto/sans/arabic/regular"
@@ -40,6 +41,32 @@ func TestWrappingTruncation(t *testing.T) {
 	}
 }
 
+// TestShapingNewlineHandling checks that the shaper's newline splitting behaves
+// consistently and does not create spurious lines of text.
+func TestShapingNewlineHandling(t *testing.T) {
+	// Use a test string containing multiple newlines to ensure that they are shaped
+	// as separate paragraphs.
+	textInput := "\n"
+	ltrFace, _ := opentype.Parse(goregular.TTF)
+	collection := []FontFace{{Face: ltrFace}}
+	cache := NewShaper(collection)
+	cache.LayoutString(Parameters{
+		Alignment: Middle,
+		PxPerEm:   fixed.I(10),
+	}, 200, 200, english, textInput)
+	if lineCount := len(cache.txt.lines); lineCount > 1 {
+		t.Errorf("shaping string %q created %d lines", textInput, lineCount)
+	}
+
+	cache.Layout(Parameters{
+		Alignment: Middle,
+		PxPerEm:   fixed.I(10),
+	}, 200, 200, english, strings.NewReader(textInput))
+	if lineCount := len(cache.txt.lines); lineCount > 1 {
+		t.Errorf("shaping reader %q created %d lines", textInput, lineCount)
+	}
+}
+
 // TestCacheEmptyString ensures that shaping the empty string returns a
 // single synthetic glyph with ascent/descent info.
 func TestCacheEmptyString(t *testing.T) {