fix(tui): address transcript selection high/medium risks

- clear stale completed selection when transcript content changes
- ignore blank viewport rows when mapping mouse selection
- preserve ansi runs outside selected range during highlight
- skip redundant redraw on unchanged drag position

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: creatang <165447160+creatang@users.noreply.github.com>

Author: xgopilot

internal/tui/core/app/copy_code.go

@@ -262,6 +262,9 @@ func (a App) selectionPositionAtMouse(msg tea.MouseMsg) (line int, col int, ok b
 	currentLine := a.transcript.YOffset + (msg.Y - y)
 	currentCol := msg.X - x
 	lines := a.selectionLines()
+	if len(lines) == 0 || currentLine < 0 || currentLine >= len(lines) {
+		return 0, 0, false
+	}
 	return a.normalizeSelectionPosition(lines, currentLine, currentCol)
 }
 
@@ -309,6 +312,9 @@ func (a *App) updateTextSelection(msg tea.MouseMsg) bool {
 	if !ok {
 		return false
 	}
+	if a.textSelection.endLine == line && a.textSelection.endCol == col {
+		return true
+	}
 	a.textSelection.endLine = line
 	a.textSelection.endCol = col
 	a.refreshTranscriptHighlight()

internal/tui/core/app/copy_code_test.go

@@ -213,6 +213,89 @@ func TestSelectionPositionAndDragGuardBranches(t *testing.T) {
 	}
 }
 
+func TestSelectionPositionAtMouseRejectsBlankViewportRows(t *testing.T) {
+	app, _ := newTestApp(t)
+	app.width = 100
+	app.height = 24
+	app.applyComponentLayout(true)
+	app.setTranscriptContent("only-one-line")
+
+	x, y, _, h := app.transcriptBounds()
+	if h < 2 {
+		t.Fatalf("expected transcript viewport with spare rows, got height=%d", h)
+	}
+
+	if _, _, ok := app.selectionPositionAtMouse(tea.MouseMsg{X: x + 1, Y: y + h - 1}); ok {
+		t.Fatalf("expected blank viewport row to be ignored")
+	}
+}
+
+func TestSetTranscriptContentClearsSelectionAfterContentChange(t *testing.T) {
+	app, _ := newTestApp(t)
+	app.width = 100
+	app.height = 24
+	app.applyComponentLayout(true)
+	app.setTranscriptContent("line-one")
+	app.textSelection.active = true
+	app.textSelection.startLine = 0
+	app.textSelection.startCol = 0
+	app.textSelection.endLine = 0
+	app.textSelection.endCol = 4
+	app.refreshTranscriptHighlight()
+
+	app.setTranscriptContent("line-two")
+	if app.textSelection.active || app.textSelection.dragging {
+		t.Fatalf("expected selection to be cleared after transcript content changes")
+	}
+	if app.hasTextSelection() {
+		t.Fatalf("expected no valid selection range after transcript content changes")
+	}
+}
+
+func TestUpdateTextSelectionSkipsUnchangedPosition(t *testing.T) {
+	app, _ := newTestApp(t)
+	app.width = 100
+	app.height = 24
+	app.applyComponentLayout(true)
+	app.setTranscriptContent("alpha\nbeta")
+
+	x, y, _, _ := app.transcriptBounds()
+	if !app.beginTextSelection(tea.MouseMsg{X: x + 1, Y: y + 1}) {
+		t.Fatalf("expected beginTextSelection to succeed")
+	}
+	if !app.updateTextSelection(tea.MouseMsg{X: x + 2, Y: y + 1}) {
+		t.Fatalf("expected first updateTextSelection to succeed")
+	}
+
+	app.transcript.SetContent("sentinel-marker")
+	if !app.updateTextSelection(tea.MouseMsg{X: x + 2, Y: y + 1}) {
+		t.Fatalf("expected unchanged motion to be handled")
+	}
+	if !strings.Contains(app.transcript.View(), "sentinel-marker") {
+		t.Fatalf("expected unchanged motion to skip redraw")
+	}
+}
+
+func TestHighlightTranscriptContentPreservesANSIOutsideSelection(t *testing.T) {
+	app, _ := newTestApp(t)
+	app.width = 100
+	app.height = 24
+	app.applyComponentLayout(true)
+	app.textSelection.active = true
+	app.textSelection.startLine = 0
+	app.textSelection.startCol = 6
+	app.textSelection.endLine = 0
+	app.textSelection.endCol = 11
+
+	highlighted := app.highlightTranscriptContent("\x1b[31mhello world\x1b[0m")
+	if !strings.Contains(highlighted, "\x1b[31m") {
+		t.Fatalf("expected highlighted content to preserve existing ANSI style runs")
+	}
+	if plain := copyCodeANSIPattern.ReplaceAllString(highlighted, ""); plain != "hello world" {
+		t.Fatalf("expected highlighted content to preserve visible text, got %q", plain)
+	}
+}
+
 func TestCopySelectionToClipboardNoSelectionNoop(t *testing.T) {
 	app, _ := newTestApp(t)
 	app.setTranscriptContent("hello")

internal/tui/core/app/update.go

@@ -2344,6 +2344,15 @@ func (a *App) rebuildTranscript() {
 
 func (a *App) setTranscriptContent(content string) {
 	normalized := normalizeTranscriptForDisplay(content)
+	contentChanged := a.transcriptContent != normalized
+	if contentChanged && a.textSelection.active && !a.textSelection.dragging {
+		a.textSelection.active = false
+		a.textSelection.dragging = false
+		a.textSelection.startLine = 0
+		a.textSelection.startCol = 0
+		a.textSelection.endLine = 0
+		a.textSelection.endCol = 0
+	}
 	a.transcriptContent = normalized
 	if a.hasTextSelection() {
 		a.transcript.SetContent(a.highlightTranscriptContent(normalized))
@@ -2364,8 +2373,7 @@ func (a *App) highlightTranscriptContent(content string) string {
 		Foreground(lipgloss.Color(selectionFg))
 
 	for i := startLine; i <= endLine && i < len(lines); i++ {
-		plain := copyCodeANSIPattern.ReplaceAllString(lines[i], "")
-		lineWidth := lipgloss.Width(plain)
+		lineWidth := ansi.StringWidth(lines[i])
 		selStart := 0
 		selEnd := lineWidth
 		if i == startLine {
@@ -2379,9 +2387,9 @@ func (a *App) highlightTranscriptContent(content string) string {
 		if selEnd <= selStart {
 			continue
 		}
-		prefix := ansi.Cut(plain, 0, selStart)
-		selected := ansi.Cut(plain, selStart, selEnd)
-		suffix := ansi.Cut(plain, selEnd, lineWidth)
+		prefix := ansi.Cut(lines[i], 0, selStart)
+		selected := ansi.Cut(lines[i], selStart, selEnd)
+		suffix := ansi.Cut(lines[i], selEnd, lineWidth)
 		lines[i] = prefix + highlightStyle.Render(selected) + suffix
 	}
 	return strings.Join(lines, "\n")

internal/tui/core/app/view_test.go

@@ -128,12 +128,12 @@ func TestRenderWaterfallThinkingState(t *testing.T) {
 func TestRenderWaterfallSelectionHint(t *testing.T) {
 	app, _ := newTestApp(t)
 	app.state.ActivePicker = pickerNone
+	app.setTranscriptContent("hello")
 	app.textSelection.active = true
 	app.textSelection.startLine = 0
 	app.textSelection.startCol = 0
 	app.textSelection.endLine = 0
 	app.textSelection.endCol = 1
-	app.setTranscriptContent("hello")
 
 	view := app.renderWaterfall(80, 24)
 	if !strings.Contains(view, "已选择内容，右键复制") {