text: fix EOF detection at newline boundaries

This commit tests and fixes some edge cases that threw off rune accounting
when a newline character was the final rune in the input *and* the text was
being truncated. I imagine they were never previously reported because it's
rare to try to truncate such text.

Thanks to Dominik Honnef for the report and reproducer.

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

text/shaper.go

@@ -316,7 +316,9 @@ func (l *Shaper) layoutText(params Parameters, txt io.Reader, str string) {
 			if !done {
 				_, re := l.reader.ReadByte()
 				done = re != nil
-				_ = l.reader.UnreadByte()
+				if !done {
+					_ = l.reader.UnreadByte()
+				}
 			}
 		} else {
 			idx := strings.IndexByte(str, '\n')
@@ -325,6 +327,7 @@ func (l *Shaper) layoutText(params Parameters, txt io.Reader, str string) {
 				endByte = len(str)
 			} else {
 				endByte = idx + 1
+				done = endByte == len(str)
 			}
 		}
 		if len(str[:endByte]) > 0 || (len(l.paragraph) > 0 || len(l.txt.lines) == 0) {

text/shaper_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	nsareg "eliasnaur.com/font/noto/sans/arabic/regular"
+	"gioui.org/font"
 	"gioui.org/font/gofont"
 	"gioui.org/font/opentype"
 	"gioui.org/io/system"
@@ -161,6 +162,7 @@ func TestShapingNewlineHandling(t *testing.T) {
 		{textInput: "a\n", expectedLines: 1, expectedGlyphs: 3},
 		{textInput: "a\nb", expectedLines: 2, expectedGlyphs: 3},
 		{textInput: "", expectedLines: 1, expectedGlyphs: 1},
+		{textInput: "\n", expectedLines: 1, expectedGlyphs: 2},
 	} {
 		t.Run(fmt.Sprintf("%q", tc.textInput), func(t *testing.T) {
 			ltrFace, _ := opentype.Parse(goregular.TTF)
@@ -168,8 +170,13 @@ func TestShapingNewlineHandling(t *testing.T) {
 			cache := NewShaper(NoSystemFonts(), WithCollection(collection))
 			checkGlyphs := func() {
 				glyphs := []Glyph{}
+				runes := 0
 				for g, ok := cache.NextGlyph(); ok; g, ok = cache.NextGlyph() {
 					glyphs = append(glyphs, g)
+					runes += g.Runes
+				}
+				if expected := len([]rune(tc.textInput)); expected != runes {
+					t.Errorf("expected %d runes, got %d", expected, runes)
 				}
 				if len(glyphs) != tc.expectedGlyphs {
 					t.Errorf("expected %d glyphs, got %d", tc.expectedGlyphs, len(glyphs))
@@ -191,8 +198,8 @@ func TestShapingNewlineHandling(t *testing.T) {
 					}
 					breakX, breakY := breakGlyph.X, breakGlyph.Y
 					startX, startY := startGlyph.X, startGlyph.Y
-					if breakX == startX {
-						t.Errorf("expected paragraph start glyph to have cursor x")
+					if breakX == startX && idx != 0 {
+						t.Errorf("expected paragraph start glyph to have cursor x, got %v", startX)
 					}
 					if breakY == startY {
 						t.Errorf("expected paragraph start glyph to have cursor y")
@@ -464,6 +471,19 @@ func TestShapeStringRuneAccounting(t *testing.T) {
 				MaxWidth: 100,
 			},
 		},
+		{
+			name:  "newline regression",
+			input: "\n",
+			params: Parameters{
+				Font:       font.Font{Typeface: "Go", Style: font.Regular, Weight: font.Normal},
+				Alignment:  Start,
+				PxPerEm:    768,
+				MaxLines:   1,
+				Truncator:  "\u200b",
+				WrapPolicy: WrapHeuristically,
+				MaxWidth:   999929,
+			},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			for _, setup := range []setup{