Fix table text copy not matching selection

When selecting text in tables, the copied content did not match
the visual selection because border characters (│, ─, etc.) were
not properly accounted for when mapping visual column positions
to text content.

The fix maps visual column positions from the rendered line (with
borders) to the stripped line (without borders) by tracking which
runes correspond to which visual columns, ensuring the copied text
matches what the user selected.

Fixes: #2167

Signed-off-by: Md Yunus <admin@yunuscollege.eu.org>

Author: yunus25jmi1

pkg/tui/components/messages/clipboard.go

@@ -112,33 +112,43 @@ func (m *model) extractSelectedText() string {
 		line := stripBorderChars(plainLine)
 		runes := []rune(line)
 
-		// Calculate how many display columns were removed by stripping border chars
-		borderOffset := runewidth.StringWidth(plainLine) - runewidth.StringWidth(line)
+		// Map visual column positions from the plain line (with borders) to the
+		// stripped line (without borders) by tracking which runes correspond to
+		// which visual columns
+		visualToRune := make(map[int]int)
+		plainRunes := []rune(plainLine)
+		visualCol := 0
+		lineRuneIdx := 0
+
+		for _, r := range plainRunes {
+			if !boxDrawingChars[r] {
+				// This rune is kept in the stripped line
+				visualToRune[visualCol] = lineRuneIdx
+				lineRuneIdx++
+			}
+			visualCol += runewidth.RuneWidth(r)
+		}
 
-		// Adjust column positions by subtracting the border offset
-		adjustedStartCol := max(0, startCol-borderOffset)
-		adjustedEndCol := max(0, endCol-borderOffset)
+		// Find the closest rune index for the start and end columns
+		startRuneIdx := findClosestRuneIndex(visualToRune, startCol, len(runes))
+		endRuneIdx := findClosestRuneIndex(visualToRune, endCol, len(runes))
 
 		var lineText string
 		switch i {
 		case startLine:
 			if startLine == endLine {
-				sIdx := displayWidthToRuneIndex(line, adjustedStartCol)
-				eIdx := min(displayWidthToRuneIndex(line, adjustedEndCol), len(runes))
-				if sIdx < len(runes) && sIdx < eIdx {
-					lineText = strings.TrimSpace(string(runes[sIdx:eIdx]))
+				if startRuneIdx < len(runes) && startRuneIdx < endRuneIdx {
+					lineText = strings.TrimSpace(string(runes[startRuneIdx:endRuneIdx]))
 				}
 				break
 			}
 			// First line: from startCol to end
-			sIdx := displayWidthToRuneIndex(line, adjustedStartCol)
-			if sIdx < len(runes) {
-				lineText = strings.TrimSpace(string(runes[sIdx:]))
+			if startRuneIdx < len(runes) {
+				lineText = strings.TrimSpace(string(runes[startRuneIdx:]))
 			}
 		case endLine:
 			// Last line: from start to endCol
-			eIdx := min(displayWidthToRuneIndex(line, adjustedEndCol), len(runes))
-			lineText = strings.TrimSpace(string(runes[:eIdx]))
+			lineText = strings.TrimSpace(string(runes[:endRuneIdx]))
 		default:
 			// Middle lines: entire line
 			lineText = strings.TrimSpace(line)
@@ -153,6 +163,32 @@ func (m *model) extractSelectedText() string {
 	return result.String()
 }
 
+// findClosestRuneIndex finds the rune index for a given visual column,
+// or the closest next rune if the exact column doesn't exist
+func findClosestRuneIndex(visualToRune map[int]int, visualCol int, maxRunes int) int {
+	// Try exact match first
+	if runeIdx, ok := visualToRune[visualCol]; ok {
+		return runeIdx
+	}
+
+	// Find the next available rune index after the visual column
+	for col := visualCol + 1; col <= visualCol+10; col++ {
+		if runeIdx, ok := visualToRune[col]; ok {
+			return runeIdx
+		}
+	}
+
+	// Find the previous available rune index
+	for col := visualCol - 1; col >= 0; col-- {
+		if runeIdx, ok := visualToRune[col]; ok {
+			return runeIdx
+		}
+	}
+
+	// Fallback: return the last rune index
+	return maxRunes
+}
+
 // copySelectionToClipboard copies the currently selected text to clipboard
 func (m *model) copySelectionToClipboard() tea.Cmd {
 	if !m.selection.active {