basicwidget: clear dropped elements in reused buffers

A buffer reused across calls keeps its dropped elements in the backing
array when truncated with s[:0], pinning every reference those elements
hold. Replace such truncations with slices.Delete, which zeroes the
removed range while keeping the capacity.

The affected buffers:

  - PieceTable.history, in both the redo truncation and resetHistory.
    Each dropped historyItem owns a cloned items slice, so a discarded
    redo state, or a previous document's entire undo history, stayed
    alive.
  - Combobox.filteredItems, refilled on every keystroke while
    filtering, stranding the strings and content widgets of the items
    that a narrowed query no longer matches.
  - tmpLocales and prevLocales in resolveFace, as language.Tag embeds
    an interface. tmpLocales kept its whole contents live after the
    function returned.
  - tableRowWidget.textColumnLayouts, the only one of the three
    buffers reset together there that was not already cleared.

appendVisualLinesFromCachedStarts had the same problem on its failure
path: it returned dst[:base] after having appended, leaving those
elements uncleared past the caller's length, out of reach of the
caller's deferred clear. Drop the redundant re-truncation in Draw as
well, which only ran when theVisualLinesBuffer was already empty.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hajimehoshi

basicwidget/combobox.go

@@ -110,7 +110,7 @@ func (c *Combobox) OnValueChanged(f func(context *guigui.Context, value string,
 
 func (c *Combobox) updateFilteredItems() {
 	input := strings.ToLower(c.textInput.Value())
-	c.filteredItems = c.filteredItems[:0]
+	c.filteredItems = slices.Delete(c.filteredItems, 0, len(c.filteredItems))
 	for _, item := range c.items {
 		if input == "" || strings.Contains(strings.ToLower(item), input) {
 			c.filteredItems = append(c.filteredItems, PopupMenuItem[string]{

basicwidget/internal/font/font.go

@@ -270,8 +270,9 @@ func resolveFace(context *guigui.Context, fnt *Family, attributes Attributes) (t
 	tmpLocales = context.AppendLocales(tmpLocales[:0])
 	if !slices.Equal(prevLocales, tmpLocales) {
 		clear(theFaceCache)
-		prevLocales = append(prevLocales[:0], tmpLocales...)
+		prevLocales = append(slices.Delete(prevLocales, 0, len(prevLocales)), tmpLocales...)
 	}
+	tmpLocales = slices.Delete(tmpLocales, 0, len(tmpLocales))
 
 	var familyID uint64
 	if fnt != nil {

basicwidget/internal/piecetable/piecetable.go

@@ -344,7 +344,7 @@ func (p *PieceTable) ReadFrom(r io.Reader) (int64, error) {
 }
 
 func (p *PieceTable) resetHistory() {
-	p.history = p.history[:0]
+	p.history = slices.Delete(p.history, 0, len(p.history))
 	p.history = append(p.history, historyItem{
 		items: []pieceTableItem{
 			{
@@ -573,7 +573,7 @@ func (p *PieceTable) maybeAppendHistory(text string, start, end int, fromIME boo
 func (p *PieceTable) appendHistory(undoStart, undoEnd, redoStart, redoEnd int) {
 	// Truncate the history.
 	if p.historyIndex < len(p.history)-1 {
-		p.history = p.history[:p.historyIndex+1]
+		p.history = slices.Delete(p.history, p.historyIndex+1, len(p.history))
 	}
 
 	// Append the current items (cloned) to the history.

basicwidget/internal/textutil/draw.go

@@ -132,7 +132,7 @@ func appendVisualLinesFromCachedStarts(dst []visualLine, str string, width int,
 		line := str[pos:lineEnd]
 		s, sok := cachedVisualLineStarts(width, line, wrapMode, face, tabWidth, keepTailingSpace)
 		if !sok {
-			return dst[:base], false
+			return slices.Delete(dst, base, len(dst)), false
 		}
 		for i := range s {
 			rs := pos + s[i]
@@ -184,7 +184,6 @@ func Draw(bounds image.Rectangle, dst *ebiten.Image, str string, options *DrawOp
 		}
 	}
 	if !built {
-		theVisualLinesBuffer = theVisualLinesBuffer[:0]
 		for vl := range visualLines(layoutWidth, str, options.WrapMode, func(str string, indexInBytes int) float64 {
 			return advance(str, indexInBytes, options.Face.TextFace(), options.TabWidth, options.KeepTailingSpace)
 		}) {

basicwidget/table.go

@@ -365,7 +365,7 @@ func (t *tableRowWidget[T]) Build(context *guigui.Context, adder *guigui.ChildAd
 
 func (t *tableRowWidget[T]) Layout(context *guigui.Context, widgetBounds *guigui.WidgetBounds, layouter *guigui.ChildLayouter) {
 	t.linearLayoutItems = slices.Delete(t.linearLayoutItems, 0, len(t.linearLayoutItems))
-	t.textColumnLayouts = t.textColumnLayouts[:0]
+	t.textColumnLayouts = slices.Delete(t.textColumnLayouts, 0, len(t.textColumnLayouts))
 	t.textColumnLayoutItems = slices.Delete(t.textColumnLayoutItems, 0, len(t.textColumnLayoutItems))
 	for i := range t.table.columnWidthsInPixels {
 		if i < len(t.row.Cells) && t.row.Cells[i].Content != nil {