widget: fix text selection and selection area rendering issues

1. When selecting multiple lines of text, the rendered selection area does not include the last character of the first line.
2. When selecting a specific line (other than the last line) in multiline text, the last character of that line cannot be selected.

Signed-off-by: CoyAce <AkeyCoy@gmail.com>

widget/index.go

@@ -22,6 +22,10 @@ type lineInfo struct {
 	glyphs          int
 }
 
+func (l lineInfo) getLineEnd() fixed.Int26_6 {
+	return l.xOff + l.width
+}
+
 type glyphIndex struct {
 	// glyphs holds the glyphs processed.
 	glyphs []text.Glyph
@@ -231,46 +235,61 @@ func (g *glyphIndex) Glyph(gl text.Glyph) {
 }
 
 func (g *glyphIndex) closestToRune(runeIdx int) (combinedPos, int) {
-	if len(g.positions) == 0 {
+	n := len(g.positions)
+	if n == 0 {
 		return combinedPos{}, 0
 	}
-	i := sort.Search(len(g.positions), func(i int) bool {
+	i := sort.Search(n, func(i int) bool {
 		pos := g.positions[i]
 		return pos.runes >= runeIdx
 	})
-	if i > 0 {
-		i--
+
+	notFound := i == n
+	if notFound {
+		return g.positions[n-1], n - 1
 	}
-	closest := g.positions[i]
-	closestI := i
-	for ; i < len(g.positions); i++ {
-		if g.positions[i].runes == runeIdx {
-			return g.positions[i], i
-		}
-	}
-	return closest, closestI
+	return g.positions[i], i
 }
 
 func (g *glyphIndex) closestToLineCol(lineCol screenPos) combinedPos {
-	if len(g.positions) == 0 {
+	n := len(g.positions)
+	if n == 0 {
 		return combinedPos{}
 	}
-	i := sort.Search(len(g.positions), func(i int) bool {
+	i := sort.Search(n, func(i int) bool {
 		pos := g.positions[i]
 		return pos.lineCol.line > lineCol.line || (pos.lineCol.line == lineCol.line && pos.lineCol.col >= lineCol.col)
 	})
-	if i > 0 {
-		i--
+	notFound := i == n
+	if notFound {
+		return g.positions[n-1]
 	}
-	prior := g.positions[i]
-	if i+1 >= len(g.positions) {
+	pos := g.positions[i]
+	foundInNextLine := pos.lineCol.line > lineCol.line
+	if foundInNextLine && i > 0 {
+		prior := g.positions[i-1]
+		prior.x = g.lines[lineCol.line].getLineEnd()
 		return prior
 	}
-	next := g.positions[i+1]
-	if next.lineCol != lineCol {
-		return prior
+	return pos
+}
+
+func (g *glyphIndex) atStartOfLine(pos combinedPos) bool {
+	if pos.runes == 0 {
+		return true
 	}
-	return next
+	prevRuneIndex := pos.runes - 1
+	lineOfPrevRune := g.positions[prevRuneIndex].lineCol.line
+	return lineOfPrevRune < pos.lineCol.line
+}
+
+func (g *glyphIndex) atEndOfLine(pos combinedPos) bool {
+	if pos.runes == g.positions[len(g.positions)-1].runes {
+		return true
+	}
+	next := pos.runes + 1
+	hasNext := next < len(g.positions)
+	return hasNext && g.positions[next].lineCol.line > pos.lineCol.line
 }
 
 func dist(a, b fixed.Int26_6) fixed.Int26_6 {
@@ -280,9 +299,9 @@ func dist(a, b fixed.Int26_6) fixed.Int26_6 {
 	return b - a
 }
 
-func (g *glyphIndex) closestToXY(x fixed.Int26_6, y int) combinedPos {
+func (g *glyphIndex) closestToXY(x fixed.Int26_6, y int) (pos combinedPos, atEndOfLine bool) {
 	if len(g.positions) == 0 {
-		return combinedPos{}
+		return combinedPos{}, false
 	}
 	i := sort.Search(len(g.positions), func(i int) bool {
 		pos := g.positions[i]
@@ -292,7 +311,7 @@ func (g *glyphIndex) closestToXY(x fixed.Int26_6, y int) combinedPos {
 	// short. Return either the last position or (if there are no
 	// positions) the zero position.
 	if i == len(g.positions) {
-		return g.positions[i-1]
+		return g.positions[i-1], false
 	}
 	first := g.positions[i]
 	// Find the best X coordinate.
@@ -308,14 +327,22 @@ func (g *glyphIndex) closestToXY(x fixed.Int26_6, y int) combinedPos {
 		distance := dist(candidate.x, x)
 		// If we are *really* close to the current position candidate, just choose it.
 		if distance.Round() == 0 {
-			return g.positions[i]
+			return g.positions[i], false
 		}
 		if distance < closestDist {
 			closestDist = distance
 			closest = i
 		}
 	}
-	return g.positions[closest]
+	next := closest + 1
+	hasNext := next < len(g.positions)
+	if hasNext && g.atEndOfLine(g.positions[closest]) {
+		distance := dist(g.lines[line].getLineEnd(), x)
+		if distance < closestDist {
+			return g.positions[next], true
+		}
+	}
+	return g.positions[closest], false
 }
 
 // makeRegion creates a text-aligned rectangle from start to end. The vertical