fix: preserve trailing zero-width caret geometry

internal/text/caret.go

@@ -58,9 +58,10 @@ func (l *Layout) visualStops(li int) []caretStop {
 			if _, ok := slices.BinarySearch(l.boundaries, at); !ok {
 				continue
 			}
-			x := x0 + (x1-x0)*float32(k)/float32(g.Runes)
+			f := line.clusterFraction(g.Cluster, g.Runes, k)
+			x := x0 + (x1-x0)*f
 			if g.RTL {
-				x = x1 - (x1-x0)*float32(k)/float32(g.Runes)
+				x = x1 - (x1-x0)*f
 			}
 			affinity, arrival := Downstream, 0
 			if k == 0 {

internal/text/caret_test.go

@@ -20,6 +20,36 @@ func mixedLayout() *Layout {
 	return l
 }
 
+func TestTrailingZeroWidthClusterGeometry(t *testing.T) {
+	runes := []rune("x\u200b")
+	shaped := shapedLine{end: len(runes), runs: []shapedRun{{
+		end: len(runes), glyphs: []Glyph{{Advance: 10}},
+	}}}
+	var y float32
+	var buf []int
+	row := line(Params{}, runes, shaped, 0, false, lineMetrics{lineHeight: 20}, &y, nil, &buf, nil)
+	l := &Layout{Runes: runes, Lines: []Line{row}}
+	for _, index := range []int{1, 2} {
+		if x, _, _ := l.Caret(index); x != 10 {
+			t.Errorf("Caret(%d) = %v, want 10", index, x)
+		}
+		for _, affinity := range []Affinity{Upstream, Downstream} {
+			if x, _, _ := l.CaretAt(CaretPosition{index, affinity}); x != 10 {
+				t.Errorf("CaretAt(%d, %v) = %v, want 10", index, affinity, x)
+			}
+		}
+	}
+	if p := l.PositionAt(10, 10); p.Index != 1 {
+		t.Errorf("hit at visible end = %+v, want index 1", p)
+	}
+	if rs := l.Selection(0, 1); len(rs) != 1 || rs[0].X != 0 || rs[0].W != 10 {
+		t.Errorf("visible selection = %+v, want full glyph advance", rs)
+	}
+	if rs := l.Selection(1, 2); len(rs) != 0 {
+		t.Errorf("zero-width selection = %+v, want no area", rs)
+	}
+}
+
 func TestVisualCaretAffinityAndNavigation(t *testing.T) {
 	l := mixedLayout()
 	a, _, _ := l.CaretAt(CaretPosition{4, Upstream})

internal/text/edit.go

@@ -11,6 +11,16 @@ import (
 // Rect is a rectangle in DIPs relative to the layout.
 type Rect struct{ X, Y, W, H float32 }
 
+// Trailing zero-width breaks may share a visible glyph's cluster, but
+// must not take part of its advance as another ligature component.
+func (line *Line) clusterFraction(start, runes, k int) float32 {
+	if end := line.zeroWidthStart; end >= 0 && start < end && start+runes > end {
+		runes = end - start
+		k = min(k, runes)
+	}
+	return float32(k) / float32(runes)
+}
+
 // lineCarets returns the x of every caret position of a line, from Start
 // to End: inside a cluster of several runes (a ligature) the positions
 // split the cluster evenly. It caches them in the line, so a layout's
@@ -39,7 +49,7 @@ func (line *Line) lineCarets() []float32 {
 		}
 	}
 	pos := func(c cluster, k int) float32 {
-		f := float32(k) / float32(c.runes)
+		f := line.clusterFraction(c.start, c.runes, k)
 		if c.rtl {
 			return c.x1 - (c.x1-c.x0)*f
 		}
@@ -182,7 +192,8 @@ func (l *Layout) AppendSelection(dst []Rect, start, end int, on bool) []Rect {
 				continue
 			}
 			width := x1 - x0
-			left, right := float32(a-first.Cluster)/float32(first.Runes), float32(z-first.Cluster)/float32(first.Runes)
+			left := line.clusterFraction(first.Cluster, first.Runes, a-first.Cluster)
+			right := line.clusterFraction(first.Cluster, first.Runes, z-first.Cluster)
 			if first.RTL {
 				left, right = 1-right, 1-left
 			}

internal/text/layout.go

@@ -371,7 +371,8 @@ type Line struct {
 	upstream []float32
 	// padding is the room the padding of spans' pieces takes in the line
 	// (Span.PaddingX).
-	padding float32
+	padding        float32
+	zeroWidthStart int
 }
 
 // Glyph is a positioned glyph.
@@ -1440,7 +1441,10 @@ func glyphCount(sl shapedLine) int {
 // offset of the layout, with its top at *y, and moves *y past it. Its
 // glyphs go to room, which has room for those of sl; buf is a buffer.
 func line(p Params, text []rune, sl shapedLine, offset int, rtl bool, m lineMetrics, y *float32, room []Glyph, buf *[]int, spans []Span) Line {
-	line := Line{Start: offset + sl.start, End: offset + sl.end, RTL: rtl, Ascent: m.ascent, Descent: m.descent}
+	line := Line{Start: offset + sl.start, End: offset + sl.end, RTL: rtl, Ascent: m.ascent, Descent: m.descent, zeroWidthStart: -1}
+	for end := sl.end; end > sl.start && text[end-1] == '\u200b'; end-- {
+		line.zeroWidthStart = offset + end - 1
+	}
 	if cap(room) > 0 {
 		line.Glyphs = room
 	}