Better support for Windows terminals

* Default border style on Windows is changed to `sharp` because some
  Windows terminals are not capable of displaying `rounded` border
  characters correctly.
* If your terminal emulator renders each box-drawing character with
  2 columns, set `RUNEWIDTH_EASTASIAN` environment variable to `1`.

8 files changed, 66 insertions, 30 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 04b276f2..f259cfb5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -88,6 +88,10 @@ CHANGELOG
       # a will put 'alpha' on the prompt, ctrl-b will put 'bravo'
       fzf --bind 'a:put+put(lpha),ctrl-b:put(bravo)'
       ```
+- Added color name `preview-label` for `--preview-label` (defaults to `label`
+  for `--border-label`)
+- Better support for (Windows) terminals where each box-drawing character
+  takes 2 columns. Set `RUNEWIDTH_EASTASIAN` environment variable to `1`.
 - Behavior changes
     - fzf will always execute the preview command if the command template
       contains `{q}` even when it's empty. If you prefer the old behavior,
@@ -114,8 +118,9 @@ CHANGELOG
       when the user manually scrolls the window, the following stops. With
       this version, fzf will resume following if the user scrolls the window
       to the bottom.
-- Added color name `preview-label` for `--preview-label` (defaults to `label`
-  for `--border-label`)
+    - Default border style on Windows is changed to `sharp` because some
+      Windows terminals are not capable of displaying `rounded` border
+      characters correctly.
 - Minor bug fixes and improvements
 
 0.35.1
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index 75856d07..6d44b461 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -230,6 +230,10 @@ Draw border around the finder
 .BR none
 .br
 
+If you use a terminal emulator where box-drawing characters take 2 columns,
+try setting \fBRUNEWIDTH_EASTASIAN\fR to \fB1\fR. If the border is still
+not properly rendered, set \fB--no-unicode\fR.
+
 .TP
 .BI "--border-label" [=LABEL]
 Label to print on the horizontal border line. Should be used with one of the
@@ -568,9 +572,9 @@ Label to print on the horizontal border line of the preview window.
 Should be used with one of the following \fB--preview-window\fR options.
 
 .br
-.B * border-rounded (default)
+.B * border-rounded (default on non-Windows platforms)
 .br
-.B * border-sharp
+.B * border-sharp (default on Windows)
 .br
 .B * border-bold
 .br
diff --git a/src/options.go b/src/options.go
index fa51cc35..fd160367 100644
--- a/src/options.go
+++ b/src/options.go
@@ -314,7 +314,7 @@ type Options struct {
 }
 
 func defaultPreviewOpts(command string) previewOpts {
-	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, false, tui.BorderRounded, 0, 0, nil}
+	return previewOpts{command, posRight, sizeSpec{50, true}, "", false, false, false, false, tui.DefaultBorderShape, 0, 0, nil}
 }
 
 func defaultOptions() *Options {
@@ -543,7 +543,7 @@ func parseBorder(str string, optional bool) tui.BorderShape {
 		return tui.BorderNone
 	default:
 		if optional && str == "" {
-			return tui.BorderRounded
+			return tui.DefaultBorderShape
 		}
 		errorExit("invalid border style (expected: rounded|sharp|bold|double|horizontal|vertical|top|bottom|left|right|none)")
 	}
diff --git a/src/terminal.go b/src/terminal.go
index 8cd9a143..41b24f4e 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -210,6 +210,7 @@ type Terminal struct {
 	window             tui.Window
 	pborder            tui.Window
 	pwindow            tui.Window
+	borderWidth        int
 	count              int
 	progress           int
 	hasLoadActions     bool
@@ -604,6 +605,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		unicode:            opts.Unicode,
 		listenPort:         opts.ListenPort,
 		borderShape:        opts.BorderShape,
+		borderWidth:        1,
 		borderLabel:        nil,
 		borderLabelOpts:    opts.BorderLabel,
 		previewLabel:       nil,
@@ -666,8 +668,11 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		}
 		t.separator, t.separatorLen = t.ansiLabelPrinter(bar, &tui.ColSeparator, true)
 	}
+	if t.unicode {
+		t.borderWidth = runewidth.RuneWidth('│')
+	}
 	if opts.Scrollbar == nil {
-		if t.unicode {
+		if t.unicode && t.borderWidth == 1 {
 			t.scrollbar = "│"
 		} else {
 			t.scrollbar = "|"
@@ -968,20 +973,21 @@ func (t *Terminal) adjustMarginAndPadding() (int, int, [4]int, [4]int) {
 		paddingInt[idx] = sizeSpecToInt(idx, sizeSpec)
 	}
 
+	bw := t.borderWidth
 	extraMargin := [4]int{} // TRBL
 	for idx, sizeSpec := range t.margin {
 		switch t.borderShape {
 		case tui.BorderHorizontal:
 			extraMargin[idx] += 1 - idx%2
 		case tui.BorderVertical:
-			extraMargin[idx] += 2 * (idx % 2)
+			extraMargin[idx] += (1 + bw) * (idx % 2)
 		case tui.BorderTop:
 			if idx == 0 {
 				extraMargin[idx]++
 			}
 		case tui.BorderRight:
 			if idx == 1 {
-				extraMargin[idx] += 2
+				extraMargin[idx] += 1 + bw
 			}
 		case tui.BorderBottom:
 			if idx == 2 {
@@ -989,10 +995,10 @@ func (t *Terminal) adjustMarginAndPadding() (int, int, [4]int, [4]int) {
 			}
 		case tui.BorderLeft:
 			if idx == 3 {
-				extraMargin[idx] += 2
+				extraMargin[idx] += 1 + bw
 			}
 		case tui.BorderRounded, tui.BorderSharp, tui.BorderBold, tui.BorderDouble:
-			extraMargin[idx] += 1 + idx%2
+			extraMargin[idx] += 1 + bw*(idx%2)
 		}
 		marginInt[idx] = sizeSpecToInt(idx, sizeSpec) + extraMargin[idx]
 	}
@@ -1106,6 +1112,7 @@ func (t *Terminal) resizeWindows(forcePreview bool) {
 				return
 			}
 			hasThreshold := previewOpts.threshold > 0 && previewOpts.alternative != nil
+			bw := t.borderWidth
 			createPreviewWindow := func(y int, x int, w int, h int) {
 				pwidth := w
 				pheight := h
@@ -1118,15 +1125,15 @@ func (t *Terminal) resizeWindows(forcePreview bool) {
 				t.pborder = t.tui.NewWindow(y, x, w, h, true, previewBorder)
 				switch previewOpts.border {
 				case tui.BorderSharp, tui.BorderRounded, tui.BorderBold, tui.BorderDouble:
-					pwidth -= 4
+					pwidth -= (1 + bw) * 2
 					pheight -= 2
-					x += 2
+					x += 1 + bw
 					y += 1
 				case tui.BorderLeft:
-					pwidth -= 2
-					x += 2
+					pwidth -= 1 + bw
+					x += 1 + bw
 				case tui.BorderRight:
-					pwidth -= 2
+					pwidth -= 1 + bw
 				case tui.BorderTop:
 					pheight -= 1
 					y += 1
@@ -1136,8 +1143,8 @@ func (t *Terminal) resizeWindows(forcePreview bool) {
 					pheight -= 2
 					y += 1
 				case tui.BorderVertical:
-					pwidth -= 4
-					x += 2
+					pwidth -= (1 + bw) * 2
+					x += 1 + bw
 				}
 				if len(t.scrollbar) > 0 && !previewOpts.border.HasRight() {
 					// Need a column to show scrollbar
@@ -1832,7 +1839,7 @@ func (t *Terminal) renderPreviewScrollbar(yoff int, barLength int, barStart int)
 	if len(t.previewer.bar) != height {
 		t.previewer.bar = make([]bool, height)
 	}
-	xshift := -2
+	xshift := -1 - t.borderWidth
 	if !t.previewOpts.border.HasRight() {
 		xshift = -1
 	}
@@ -1846,7 +1853,7 @@ func (t *Terminal) renderPreviewScrollbar(yoff int, barLength int, barStart int)
 
 		// Avoid unnecessary redraws
 		bar := i >= yoff+barStart && i < yoff+barStart+barLength
-		if bar == t.previewer.bar[i] {
+		if bar == t.previewer.bar[i] && !t.tui.NeedScrollbarRedraw() {
 			continue
 		}
 
diff --git a/src/tui/dummy.go b/src/tui/dummy.go
index adecd6fc..7a02a8af 100644
--- a/src/tui/dummy.go
+++ b/src/tui/dummy.go
@@ -8,6 +8,8 @@ func HasFullscreenRenderer() bool {
 	return false
 }
 
+var DefaultBorderShape BorderShape = BorderRounded
+
 func (a Attr) Merge(b Attr) Attr {
 	return a | b
 }
@@ -32,6 +34,7 @@ func (r *FullscreenRenderer) Resize(maxHeightFunc func(int) int) {}
 func (r *FullscreenRenderer) Pause(bool)                         {}
 func (r *FullscreenRenderer) Resume(bool, bool)                  {}
 func (r *FullscreenRenderer) Clear()                             {}
+func (r *FullscreenRenderer) NeedScrollbarRedraw() bool          { return false }
 func (r *FullscreenRenderer) Refresh()                           {}
 func (r *FullscreenRenderer) Close()                             {}
 
diff --git a/src/tui/light.go b/src/tui/light.go
index c41de480..578118b7 100644
--- a/src/tui/light.go
+++ b/src/tui/light.go
@@ -651,6 +651,10 @@ func (r *LightRenderer) Clear() {
 	r.flush()
 }
 
+func (r *LightRenderer) NeedScrollbarRedraw() bool {
+	return false
+}
+
 func (r *LightRenderer) RefreshWindows(windows []Window) {
 	r.flush()
 }
@@ -743,13 +747,14 @@ func (w *LightWindow) drawBorderHorizontal(top, bottom bool) {
 	if w.preview {
 		color = ColPreviewBorder
 	}
+	hw := runewidth.RuneWidth(w.border.horizontal)
 	if top {
 		w.Move(0, 0)
-		w.CPrint(color, repeat(w.border.horizontal, w.width))
+		w.CPrint(color, repeat(w.border.horizontal, w.width/hw))
 	}
 	if bottom {
 		w.Move(w.height-1, 0)
-		w.CPrint(color, repeat(w.border.horizontal, w.width))
+		w.CPrint(color, repeat(w.border.horizontal, w.width/hw))
 	}
 }
 
@@ -780,15 +785,19 @@ func (w *LightWindow) drawBorderAround() {
 	if w.preview {
 		color = ColPreviewBorder
 	}
-	w.CPrint(color, string(w.border.topLeft)+repeat(w.border.horizontal, w.width-2)+string(w.border.topRight))
+	hw := runewidth.RuneWidth(w.border.horizontal)
+	vw := runewidth.RuneWidth(w.border.vertical)
+	tcw := runewidth.RuneWidth(w.border.topLeft) + runewidth.RuneWidth(w.border.topRight)
+	bcw := runewidth.RuneWidth(w.border.bottomLeft) + runewidth.RuneWidth(w.border.bottomRight)
+	w.CPrint(color, string(w.border.topLeft)+repeat(w.border.horizontal, (w.width-tcw)/hw)+string(w.border.topRight))
 	for y := 1; y < w.height-1; y++ {
 		w.Move(y, 0)
 		w.CPrint(color, string(w.border.vertical))
-		w.CPrint(color, repeat(' ', w.width-2))
+		w.CPrint(color, repeat(' ', w.width-vw*2))
 		w.CPrint(color, string(w.border.vertical))
 	}
 	w.Move(w.height-1, 0)
-	w.CPrint(color, string(w.border.bottomLeft)+repeat(w.border.horizontal, w.width-2)+string(w.border.bottomRight))
+	w.CPrint(color, string(w.border.bottomLeft)+repeat(w.border.horizontal, (w.width-bcw)/hw)+string(w.border.bottomRight))
 }
 
 func (w *LightWindow) csi(code string) {
diff --git a/src/tui/tcell.go b/src/tui/tcell.go
index 82d72995..f1f18e24 100644
--- a/src/tui/tcell.go
+++ b/src/tui/tcell.go
@@ -17,6 +17,8 @@ func HasFullscreenRenderer() bool {
 	return true
 }
 
+var DefaultBorderShape BorderShape = BorderSharp
+
 func asTcellColor(color Color) tcell.Color {
 	if color == colDefault {
 		return tcell.ColorDefault
@@ -187,6 +189,10 @@ func (r *FullscreenRenderer) Clear() {
 	_screen.Clear()
 }
 
+func (r *FullscreenRenderer) NeedScrollbarRedraw() bool {
+	return true
+}
+
 func (r *FullscreenRenderer) Refresh() {
 	// noop
 }
@@ -686,15 +692,16 @@ func (w *TcellWindow) drawBorder() {
 		style = w.normal.style()
 	}
 
+	hw := runewidth.RuneWidth(w.borderStyle.horizontal)
 	switch shape {
 	case BorderRounded, BorderSharp, BorderBold, BorderDouble, BorderHorizontal, BorderTop:
-		for x := left; x < right; x++ {
+		for x := left; x <= right-hw; x += hw {
 			_screen.SetContent(x, top, w.borderStyle.horizontal, nil, style)
 		}
 	}
 	switch shape {
 	case BorderRounded, BorderSharp, BorderBold, BorderDouble, BorderHorizontal, BorderBottom:
-		for x := left; x < right; x++ {
+		for x := left; x <= right-hw; x += hw {
 			_screen.SetContent(x, bot-1, w.borderStyle.horizontal, nil, style)
 		}
 	}
@@ -707,14 +714,14 @@ func (w *TcellWindow) drawBorder() {
 	switch shape {
 	case BorderRounded, BorderSharp, BorderBold, BorderDouble, BorderVertical, BorderRight:
 		for y := top; y < bot; y++ {
-			_screen.SetContent(right-1, y, w.borderStyle.vertical, nil, style)
+			_screen.SetContent(right-hw, y, w.borderStyle.vertical, nil, style)
 		}
 	}
 	switch shape {
 	case BorderRounded, BorderSharp, BorderBold, BorderDouble:
 		_screen.SetContent(left, top, w.borderStyle.topLeft, nil, style)
-		_screen.SetContent(right-1, top, w.borderStyle.topRight, nil, style)
+		_screen.SetContent(right-runewidth.RuneWidth(w.borderStyle.topRight), top, w.borderStyle.topRight, nil, style)
 		_screen.SetContent(left, bot-1, w.borderStyle.bottomLeft, nil, style)
-		_screen.SetContent(right-1, bot-1, w.borderStyle.bottomRight, nil, style)
+		_screen.SetContent(right-runewidth.RuneWidth(w.borderStyle.bottomRight), bot-1, w.borderStyle.bottomRight, nil, style)
 	}
 }
diff --git a/src/tui/tui.go b/src/tui/tui.go
index c6a65d82..5a86453b 100644
--- a/src/tui/tui.go
+++ b/src/tui/tui.go
@@ -410,6 +410,7 @@ type Renderer interface {
 	RefreshWindows(windows []Window)
 	Refresh()
 	Close()
+	NeedScrollbarRedraw() bool
 
 	GetChar() Event