go.*,text,widget{,/material}: implement text truncators

This commit adds support for the idea of a text "Truncator", a string
that is shown at the end of truncated text to indicate that it has been
shortened because it would not fit within the requested number of lines.

When specifying a maximum number of lines, a truncator symbol is always
used. If the user does not provide one, the rune `…` is used. This
requirement results in a better user experience and significantly simpler
code, as we can rely upon the presence of one or more truncator glyphs in
the output glyph stream when truncation has occurred.

When interacting with truncated text, the truncator glyphs all act as
a single, indivisible unit. They can be selected or not, and if selected
they act as the entire contents of the truncated portion of the text.
This means that copying all of a truncated label will copy the entire
label text content, with the truncator symbol not appearing at all.

Concretely, the exposed text API now accepts a Truncator string in
text.Parameters, and there is a new glyph flag FlagTruncator which indicates
that the glyph is part of the truncator run. The truncator run will only
have a single FlagClusterBreak (even if the run would usually have many),
and the glyph with both FlagClusterBreak and FlagTruncator will have the
quantity of truncated runes in its Runes field. This necessitated increasing
the size of the Runes field from a byte to an int, as it's theoretically possible
for quite a lot of text to be truncated.

This commit necessarily bumps our go-text/typesetting dependency to the version
exposing truncation in the exported API.

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

text/shaper.go

@@ -27,6 +27,10 @@ type Parameters struct {
 	PxPerEm fixed.Int26_6
 	// MaxLines limits the quantity of shaped lines. Zero means no limit.
 	MaxLines int
+	// Truncator is a string of text to insert where the shaped text was truncated, which
+	// can currently ohly happen if MaxLines is nonzero and the text on the final line is
+	// truncated.
+	Truncator string
 }
 
 // A FontFace is a Font and a matching Face.
@@ -76,7 +80,7 @@ type Glyph struct {
 	// belongs to. If Flags does not contain FlagClusterBreak, this value will
 	// always be zero. The final glyph in the cluster contains the runes count
 	// for the entire cluster.
-	Runes byte
+	Runes int
 	// Flags encode special properties of this glyph.
 	Flags Flags
 }
@@ -105,6 +109,11 @@ const (
 	FlagParagraphBreak
 	// FlagParagraphStart indicates that the glyph starts a new paragraph.
 	FlagParagraphStart
+	// FlagTruncator indicates that the glyph is part of a special truncator run that
+	// represents the portion of text removed due to truncation. A glyph with both
+	// FlagTruncator and FlagClusterBreak will have a Runes field accounting for all
+	// runes truncated.
+	FlagTruncator
 )
 
 func (f Flags) String() string {
@@ -139,6 +148,11 @@ func (f Flags) String() string {
 	} else {
 		b.WriteString("_")
 	}
+	if f&FlagTruncator != 0 {
+		b.WriteString("…")
+	} else {
+		b.WriteString("_")
+	}
 	return b.String()
 }
 
@@ -206,7 +220,6 @@ func (l *Shaper) layoutText(params Parameters, minWidth, maxWidth int, lc system
 		return
 	}
 	truncating := params.MaxLines > 0
-	maxLines := params.MaxLines
 	var done bool
 	var startByte int
 	var endByte int
@@ -237,13 +250,33 @@ func (l *Shaper) layoutText(params Parameters, minWidth, maxWidth int, lc system
 			done = endByte == len(str)
 		}
 		if startByte != endByte || (len(l.paragraph) > 0 || len(l.txt.lines) == 0) {
-			l.txt.append(l.layoutParagraph(params, minWidth, maxWidth, lc, str[startByte:endByte], l.paragraph))
+			lines := l.layoutParagraph(params, minWidth, maxWidth, lc, str[startByte:endByte], l.paragraph)
 			if truncating {
-				params.MaxLines = maxLines - len(l.txt.lines)
+				params.MaxLines -= len(lines.lines)
 				if params.MaxLines == 0 {
 					done = true
+					// We've truncated the text, but we need to account for all of the runes we never
+					// decoded in the truncator.
+					var unreadRunes int
+					if txt == nil {
+						unreadRunes = utf8.RuneCountInString(str[endByte:])
+					} else {
+						for {
+							_, _, e := txt.ReadRune()
+							if e != nil {
+								break
+							}
+							unreadRunes++
+						}
+					}
+					lastLineIdx := len(lines.lines) - 1
+					lastRunIdx := len(lines.lines[lastLineIdx].runs) - 1
+					lastGlyphIdx := len(lines.lines[lastLineIdx].runs[lastRunIdx].Glyphs) - 1
+					lines.lines[lastLineIdx].runs[lastRunIdx].Runes.Count += unreadRunes
+					lines.lines[lastLineIdx].runs[lastRunIdx].Glyphs[lastGlyphIdx].runeCount += unreadRunes
 				}
 			}
+			l.txt.append(lines)
 		}
 		if done {
 			return
@@ -261,13 +294,14 @@ func (l *Shaper) layoutParagraph(params Parameters, minWidth, maxWidth int, lc s
 	}
 	// Alignment is not part of the cache key because changing it does not impact shaping.
 	lk := layoutKey{
-		ppem:     params.PxPerEm,
-		maxWidth: maxWidth,
-		minWidth: minWidth,
-		maxLines: params.MaxLines,
-		str:      asStr,
-		locale:   lc,
-		font:     params.Font,
+		truncator: params.Truncator,
+		ppem:      params.PxPerEm,
+		maxWidth:  maxWidth,
+		minWidth:  minWidth,
+		maxLines:  params.MaxLines,
+		str:       asStr,
+		locale:    lc,
+		font:      params.Font,
 	}
 	if l, ok := l.layoutCache.Get(lk); ok {
 		return l
@@ -349,13 +383,16 @@ func (l *Shaper) NextGlyph() (_ Glyph, ok bool) {
 			Ascent:  line.ascent,
 			Descent: line.descent,
 			Advance: g.xAdvance,
-			Runes:   byte(g.runeCount),
+			Runes:   g.runeCount,
 			Offset: fixed.Point26_6{
 				X: g.xOffset,
 				Y: g.yOffset,
 			},
 			Bounds: g.bounds,
 		}
+		if run.truncator {
+			glyph.Flags |= FlagTruncator
+		}
 		l.glyph++
 		if !rtl {
 			l.advance += g.xAdvance
@@ -375,6 +412,10 @@ func (l *Shaper) NextGlyph() (_ Glyph, ok bool) {
 			nextGlyph = len(run.Glyphs) - 1 - nextGlyph
 		}
 		endOfCluster := endOfRun || run.Glyphs[nextGlyph].clusterIndex != g.clusterIndex
+		if run.truncator {
+			// Only emit a single cluster for the entire truncator sequence.
+			endOfCluster = endOfRun
+		}
 		if endOfCluster {
 			glyph.Flags |= FlagClusterBreak
 		} else {
@@ -404,7 +445,6 @@ func (l *Shaper) NextGlyph() (_ Glyph, ok bool) {
 				l.pararagraphStart.Y = glyph.Y + int32((glyph.Ascent + glyph.Descent).Ceil())
 			}
 		}
-
 		return glyph, true
 	}
 }