Fix nth attr merge order to respect precedence hierarchy (#4697)

* Fix nth attr merge order to respect precedence hierarchy

nth attrs were merged ON TOP of current-fg/selected-fg attrs, so
nth:regular would clear attrs like underline from current-fg. Fix the
merge chain to apply nth BEFORE the line-type overlay:

  fg < nth < selected-fg < current-fg < hl < selected-hl < current-hl

Store raw current-fg and selected-fg attrs in ColorTheme before they
get merged with fg/ListFg, then pass them as nthOverlay through
printHighlighted to colorOffsets where the correct merge chain is
computed: fgAttr.Merge(nthAttr).Merge(nthOverlay).

Fix #4687

* Make current-fg inherit from list-fg instead of fg

current-fg was inheriting from fg, not list-fg, so setting list-fg
had no effect on the current line. e.g.

  fzf --color fg:dim,list-fg:underline,nth:regular -d / --nth -1

The non-nth part of the current line was dim (from fg) instead of
underline (from list-fg). Resolve ListFg/ListBg earlier in InitTheme
so Current can inherit from them.

* Make selected-fg inherit from list-fg via merge instead of override

selected-fg used o() which replaces the attr from list-fg entirely.
e.g. with fg:dim,selected-fg:italic, the dim was lost on selected
lines because o() replaced dim with italic instead of merging them.
Use ColorAttr.Merge() so attrs are combined additively, consistent
with how current-fg inherits from list-fg.

* Apply selected-fg attrs on current line when item is selected

When an item is both current and selected, the current-line rendering
ignored selected-fg attrs entirely. e.g.

  echo foo | fzf --color fg:dim,nth:regular,current-fg:underline,selected-fg:italic --bind result:select --multi

The italic from selected-fg was lost. Merge NthSelectedAttr into the
overlay and rebuild colBase attr with the correct precedence chain:
fg < selected-fg < current-fg.

src/result.go

@@ -128,7 +128,7 @@ func minRank() Result {
 	return Result{item: &minItem, points: [4]uint16{math.MaxUint16, 0, 0, 0}}
 }
 
-func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, theme *tui.ColorTheme, colBase tui.ColorPair, colMatch tui.ColorPair, attrNth tui.Attr, hidden bool) []colorOffset {
+func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, theme *tui.ColorTheme, colBase tui.ColorPair, colMatch tui.ColorPair, attrNth tui.Attr, nthOverlay tui.Attr, hidden bool) []colorOffset {
 	itemColors := result.item.Colors()
 
 	// No ANSI codes
@@ -213,6 +213,10 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 		}
 		return tui.NewColorPair(fg, bg, ansi.color.attr).WithUl(ansi.color.ul).MergeAttr(base)
 	}
+	fgAttr := tui.ColNormal.Attr()
+	nthAttrFinal := fgAttr.Merge(attrNth).Merge(nthOverlay)
+	nthBase := colBase.WithNewAttr(nthAttrFinal)
+
 	var colors []colorOffset
 	add := func(idx int) {
 		if curr.fbg >= 0 {
@@ -226,7 +230,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 			if curr.match {
 				var color tui.ColorPair
 				if curr.nth {
-					color = colBase.WithAttr(attrNth).Merge(colMatch)
+					color = nthBase.Merge(colMatch)
 				} else {
 					color = colBase.Merge(colMatch)
 				}
@@ -246,7 +250,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 					if color.Fg().IsDefault() && origColor.HasBg() {
 						color = origColor
 						if curr.nth {
-							color = color.WithAttr(attrNth &^ tui.AttrRegular)
+							color = color.WithAttr((attrNth &^ tui.AttrRegular).Merge(nthOverlay))
 						}
 					} else {
 						color = origColor.MergeNonDefault(color)
@@ -258,7 +262,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 				ansi := itemColors[curr.index]
 				base := colBase
 				if curr.nth {
-					base = base.WithAttr(attrNth)
+					base = nthBase
 				}
 				if hidden {
 					base = base.WithFg(theme.Nomatch)
@@ -270,7 +274,7 @@ func (result *Result) colorOffsets(matchOffsets []Offset, nthOffsets []Offset, t
 					match:  false,
 					url:    ansi.color.url})
 			} else {
-				color := colBase.WithAttr(attrNth)
+				color := nthBase
 				if hidden {
 					color = color.WithFg(theme.Nomatch)
 				}

src/result_test.go

@@ -132,7 +132,7 @@ func TestColorOffset(t *testing.T) {
 
 	colBase := tui.NewColorPair(89, 189, tui.AttrUndefined)
 	colMatch := tui.NewColorPair(99, 199, tui.AttrUndefined)
-	colors := item.colorOffsets(offsets, nil, tui.Dark256, colBase, colMatch, tui.AttrUndefined, false)
+	colors := item.colorOffsets(offsets, nil, tui.Dark256, colBase, colMatch, tui.AttrUndefined, 0, false)
 	assert := func(idx int, b int32, e int32, c tui.ColorPair) {
 		o := colors[idx]
 		if o.offset[0] != b || o.offset[1] != e || o.color != c {
@@ -159,7 +159,7 @@ func TestColorOffset(t *testing.T) {
 
 	nthOffsets := []Offset{{37, 39}, {42, 45}}
 	for _, attr := range []tui.Attr{tui.AttrRegular, tui.StrikeThrough} {
-		colors = item.colorOffsets(offsets, nthOffsets, tui.Dark256, colRegular, colUnderline, attr, false)
+		colors = item.colorOffsets(offsets, nthOffsets, tui.Dark256, colRegular, colUnderline, attr, 0, false)
 
 		// [{[0 5] {1 5 0}} {[5 15] {1 5 8}} {[15 20] {1 5 0}}
 		//  {[22 25] {2 6 1}} {[25 27] {2 6 9}} {[27 30] {-1 -1 8}}
@@ -182,6 +182,37 @@ func TestColorOffset(t *testing.T) {
 		assert(10, 37, 39, tui.NewColorPair(4, 8, expected))
 		assert(11, 39, 40, tui.NewColorPair(4, 8, tui.Bold))
 	}
+
+	// Test nthOverlay: simulates nth:regular with current-fg:underline
+	// The overlay (underline) should survive even though nth:regular clears attrs.
+	// Precedence: fg < nth < current-fg
+	colors = item.colorOffsets(offsets, nthOffsets, tui.Dark256, colRegular, colUnderline, tui.AttrRegular, tui.Underline, false)
+
+	// nth regions should have Underline (from overlay), not cleared by AttrRegular
+	// Non-nth regions keep colBase attrs (AttrUndefined)
+	assert(0, 0, 5, tui.NewColorPair(1, 5, tui.AttrUndefined))
+	assert(1, 5, 15, tui.NewColorPair(1, 5, tui.Underline))
+	assert(2, 15, 20, tui.NewColorPair(1, 5, tui.AttrUndefined))
+	assert(3, 22, 25, tui.NewColorPair(2, 6, tui.Bold))
+	assert(4, 25, 27, tui.NewColorPair(2, 6, tui.Bold|tui.Underline))
+	assert(5, 27, 30, colUnderline)
+	assert(6, 30, 32, tui.NewColorPair(3, 7, tui.Underline))
+	assert(7, 32, 33, colUnderline)
+	assert(8, 33, 35, tui.NewColorPair(4, 8, tui.Bold|tui.Underline))
+	assert(9, 35, 37, tui.NewColorPair(4, 8, tui.Bold))
+	// nth region within ANSI bold: AttrRegular clears, overlay adds Underline back
+	assert(10, 37, 39, tui.NewColorPair(4, 8, tui.Bold|tui.Underline))
+	assert(11, 39, 40, tui.NewColorPair(4, 8, tui.Bold))
+
+	// Test nthOverlay with additive attrs: nth:strikethrough with selected-fg:bold
+	colors = item.colorOffsets(offsets, nthOffsets, tui.Dark256, colRegular, colUnderline, tui.StrikeThrough, tui.Bold, false)
+
+	// Non-nth entries unchanged from overlay=0 case
+	assert(0, 0, 5, tui.NewColorPair(1, 5, tui.AttrUndefined))
+	assert(5, 27, 30, colUnderline) // match only, no nth
+	assert(7, 32, 33, colUnderline) // match only, no nth
+	// nth region within ANSI bold: StrikeThrough|Bold merged with ANSI Bold
+	assert(10, 37, 39, tui.NewColorPair(4, 8, tui.Bold|tui.StrikeThrough))
 }
 
 func TestRadixSortResults(t *testing.T) {

src/terminal.go

@@ -1554,7 +1554,7 @@ func (t *Terminal) ansiLabelPrinter(str string, color *tui.ColorPair, fill bool)
 	printFn := func(window tui.Window, limit int) {
 		if offsets == nil {
 			// tui.Col* are not initialized until renderer.Init()
-			offsets = result.colorOffsets(nil, nil, t.theme, *color, *color, t.nthAttr, false)
+			offsets = result.colorOffsets(nil, nil, t.theme, *color, *color, t.nthAttr, 0, false)
 		}
 		for limit > 0 {
 			if length > limit {
@@ -1617,7 +1617,7 @@ func (t *Terminal) parsePrompt(prompt string) (func(), int) {
 				return 1
 			}
 			t.printHighlighted(
-				Result{item: item}, tui.ColPrompt, tui.ColPrompt, false, false, false, line, line, true, preTask, nil)
+				Result{item: item}, tui.ColPrompt, tui.ColPrompt, false, false, false, line, line, true, preTask, nil, 0)
 		})
 		t.wrap = wrap
 	}
@@ -3185,7 +3185,7 @@ func (t *Terminal) printFooter() {
 				func(markerClass) int {
 					t.footerWindow.Print(indent)
 					return indentSize
-				}, nil)
+				}, nil, 0)
 		}
 	})
 	t.wrap = wrap
@@ -3269,7 +3269,7 @@ func (t *Terminal) printHeaderImpl(window tui.Window, borderShape tui.BorderShap
 			func(markerClass) int {
 				t.window.Print(indent)
 				return indentSize
-			}, nil)
+			}, nil, 0)
 	}
 	t.wrap = wrap
 }
@@ -3507,7 +3507,14 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 			}
 			return indentSize
 		}
-		finalLineNum = t.printHighlighted(result, tui.ColCurrent, tui.ColCurrentMatch, true, true, !matched, line, maxLine, forceRedraw, preTask, postTask)
+		colCurrent := tui.ColCurrent
+		nthOverlay := t.theme.NthCurrentAttr
+		if selected {
+			nthOverlay = t.theme.NthSelectedAttr.Merge(t.theme.NthCurrentAttr)
+			baseAttr := tui.ColNormal.Attr().Merge(t.theme.NthSelectedAttr).Merge(t.theme.NthCurrentAttr)
+			colCurrent = colCurrent.WithNewAttr(baseAttr)
+		}
+		finalLineNum = t.printHighlighted(result, colCurrent, tui.ColCurrentMatch, true, true, !matched, line, maxLine, forceRedraw, preTask, postTask, nthOverlay)
 	} else {
 		preTask := func(marker markerClass) int {
 			w := t.window.Width() - t.pointerLen
@@ -3541,7 +3548,11 @@ func (t *Terminal) printItem(result Result, line int, maxLine int, index int, cu
 			base = base.WithBg(altBg)
 			match = match.WithBg(altBg)
 		}
-		finalLineNum = t.printHighlighted(result, base, match, false, true, !matched, line, maxLine, forceRedraw, preTask, postTask)
+		var nthOverlay tui.Attr
+		if selected {
+			nthOverlay = t.theme.NthSelectedAttr
+		}
+		finalLineNum = t.printHighlighted(result, base, match, false, true, !matched, line, maxLine, forceRedraw, preTask, postTask, nthOverlay)
 	}
 	for i := 0; i < t.gap && finalLineNum < maxLine; i++ {
 		finalLineNum++
@@ -3642,7 +3653,7 @@ func (t *Terminal) overflow(runes []rune, max int) bool {
 	return t.displayWidthWithLimit(runes, 0, max) > max
 }
 
-func (t *Terminal) printHighlighted(result Result, colBase tui.ColorPair, colMatch tui.ColorPair, current bool, match bool, hidden bool, lineNum int, maxLineNum int, forceRedraw bool, preTask func(markerClass) int, postTask func(int, int, bool, bool, tui.ColorPair)) int {
+func (t *Terminal) printHighlighted(result Result, colBase tui.ColorPair, colMatch tui.ColorPair, current bool, match bool, hidden bool, lineNum int, maxLineNum int, forceRedraw bool, preTask func(markerClass) int, postTask func(int, int, bool, bool, tui.ColorPair), nthOverlay tui.Attr) int {
 	var displayWidth int
 	item := result.item
 	matchOffsets := []Offset{}
@@ -3683,7 +3694,9 @@ func (t *Terminal) printHighlighted(result Result, colBase tui.ColorPair, colMat
 			// But if 'nth' is set to 'regular', it's a sign that you're applying
 			// a different style to the rest of the string. e.g. 'nth:regular,fg:dim'
 			// In this case, we still need to apply it to clear the style.
-			colBase = colBase.WithAttr(t.nthAttr)
+			fgAttr := tui.ColNormal.Attr()
+			nthAttrFinal := fgAttr.Merge(t.nthAttr).Merge(nthOverlay)
+			colBase = colBase.WithNewAttr(nthAttrFinal)
 		}
 		if !wholeCovered && t.nthAttr > 0 {
 			var tokens []Token
@@ -3702,7 +3715,7 @@ func (t *Terminal) printHighlighted(result Result, colBase tui.ColorPair, colMat
 			sort.Sort(ByOrder(nthOffsets))
 		}
 	}
-	allOffsets := result.colorOffsets(charOffsets, nthOffsets, t.theme, colBase, colMatch, t.nthAttr, hidden)
+	allOffsets := result.colorOffsets(charOffsets, nthOffsets, t.theme, colBase, colMatch, t.nthAttr, nthOverlay, hidden)
 
 	// Determine split offset for horizontal scrolling with freeze
 	splitOffset1 := -1

src/tui/tui.go

@@ -447,6 +447,12 @@ func (p ColorPair) WithAttr(attr Attr) ColorPair {
 	return dup
 }
 
+func (p ColorPair) WithNewAttr(attr Attr) ColorPair {
+	dup := p
+	dup.attr = attr
+	return dup
+}
+
 func (p ColorPair) WithFg(fg ColorAttr) ColorPair {
 	dup := p
 	fgPair := ColorPair{fg.Color, colUndefined, colUndefined, fg.Attr}
@@ -520,6 +526,8 @@ type ColorTheme struct {
 	ListLabel        ColorAttr
 	ListBorder       ColorAttr
 	GapLine          ColorAttr
+	NthCurrentAttr   Attr // raw current-fg attr (before fg merge) for nth overlay
+	NthSelectedAttr  Attr // raw selected-fg attr (before ListFg inherit) for nth overlay
 }
 
 type Event struct {
@@ -1199,13 +1207,19 @@ func InitTheme(theme *ColorTheme, baseTheme *ColorTheme, boldify bool, forceBlac
 		match.Attr = Underline
 	}
 	theme.Match = o(baseTheme.Match, match)
-	// Inherit from 'fg', so that we don't have to write 'current-fg:dim'
+	// These colors are not defined in the base themes.
+	// Resolve ListFg/ListBg early so Current and Selected can inherit from them.
+	theme.ListFg = o(theme.Fg, theme.ListFg)
+	theme.ListBg = o(theme.Bg, theme.ListBg)
+	// Inherit from 'list-fg', so that we don't have to write 'current-fg:dim'
 	// e.g. fzf --delimiter / --nth -1 --color fg:dim,nth:regular
 	current := theme.Current
 	if !baseTheme.Colored && current.IsUndefined() {
 		current.Attr |= Reverse
 	}
-	theme.Current = theme.Fg.Merge(o(baseTheme.Current, current))
+	resolvedCurrent := o(baseTheme.Current, current)
+	theme.NthCurrentAttr = resolvedCurrent.Attr
+	theme.Current = theme.ListFg.Merge(resolvedCurrent)
 	currentMatch := theme.CurrentMatch
 	if !baseTheme.Colored && currentMatch.IsUndefined() {
 		currentMatch.Attr |= Reverse | Underline
@@ -1230,10 +1244,8 @@ func InitTheme(theme *ColorTheme, baseTheme *ColorTheme, boldify bool, forceBlac
 	scrollbarDefined := theme.Scrollbar != undefined
 	previewBorderDefined := theme.PreviewBorder != undefined
 
-	// These colors are not defined in the base themes
+	theme.NthSelectedAttr = theme.SelectedFg.Attr
-	theme.ListFg = o(theme.Fg, theme.ListFg)
+	theme.SelectedFg = theme.ListFg.Merge(theme.SelectedFg)
-	theme.ListBg = o(theme.Bg, theme.ListBg)
-	theme.SelectedFg = o(theme.ListFg, theme.SelectedFg)
 	theme.SelectedBg = o(theme.ListBg, theme.SelectedBg)
 	theme.SelectedMatch = o(theme.Match, theme.SelectedMatch)