widget: update glyph iteration

This commit updates the textIterator and glyphIndex types to consume
new flag information provided on glyphs. These changes allow widget.Label
and widget.Editor to correctly compute text bounding boxes and to
generate valid cursor positions at the end of text.

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

widget/index.go

@@ -108,7 +108,7 @@ func (g *glyphIndex) Glyph(gl text.Glyph) {
 	if end := gl.X + gl.Advance; end > g.currentLineMax {
 		g.currentLineMax = end
 	}
-	if !g.skipPrior || gl.Flags&text.FlagTowardOrigin != g.prog {
+	if !g.skipPrior || gl.Flags&text.FlagTowardOrigin != g.prog || gl.Flags&text.FlagParagraphStart != 0 {
 		// Set the new text progression based on that of the first glyph.
 		g.prog = gl.Flags & text.FlagTowardOrigin
 		g.pos.towardOrigin = g.prog == text.FlagTowardOrigin
@@ -127,19 +127,19 @@ func (g *glyphIndex) Glyph(gl text.Glyph) {
 	}
 	needsNewLine := gl.Flags&text.FlagLineBreak > 0
 	needsNewRun := gl.Flags&text.FlagRunBreak > 0
+	breaksParagraph := gl.Flags&text.FlagParagraphBreak > 0
 
 	// We should insert new positions if the glyph we're processing terminates
 	// a glyph cluster.
-	insertPositionAfter := gl.Flags&text.FlagClusterBreak > 0
-	if gl.Flags&text.FlagSynthetic > 0 {
-		// Synthetic clusters shouldn't have positions generated for both
+	insertPositionAfter := gl.Flags&text.FlagClusterBreak > 0 && !breaksParagraph && gl.Runes > 0
+	if breaksParagraph {
+		// Paragraph breaking clusters shouldn't have positions generated for both
 		// sides of them. They're always zero-width, so doing so would
 		// create two visually identical cursor positions. Just reset
 		// cluster state, increment by their runes, and move on to the
 		// next glyph.
 		g.clusterAdvance = 0
 		g.pos.runes += int(gl.Runes)
-		insertPositionAfter = false
 	}
 	// Always track the cumulative advance added by the glyph, even if it
 	// doesn't terminate a cluster itself.