feat!(border): left/right borders may now be set without top/bottom

If the top and bottom borders are missing the left and right borders
will now render. If the text value rendered is one line, this will
essentially work akin to inline mode.

borders.go

@@ -11,14 +11,17 @@ import (
 // Border contains a series of values which comprise the various parts of a
 // border.
 type Border struct {
-	Top          string
-	Bottom       string
-	Left         string
-	Right        string
-	TopLeft      string
-	TopRight     string
-	BottomLeft   string
-	BottomRight  string
+	// Values for all borders.
+	Top         string
+	Bottom      string
+	Left        string
+	Right       string
+	TopLeft     string
+	TopRight    string
+	BottomLeft  string
+	BottomRight string
+
+	// Values for table borders.
 	MiddleLeft   string
 	MiddleRight  string
 	Middle       string
@@ -235,17 +238,26 @@ func (s Style) applyBorder(str string) string {
 
 		border = s.getBorderStyle()
 
+		hasBorderTopLeft     = border.TopLeft != ""
+		hasBorderTop         = border.Top != ""
+		hasBorderTopRight    = border.TopRight != ""
+		hasBorderRight       = border.Right != ""
+		hasBorderBottomRight = border.BottomRight != ""
+		hasBorderBottom      = border.Bottom != ""
+		hasBorderBottomLeft  = border.BottomLeft != ""
+		hasBorderLeft        = border.Left != ""
+
 		// Determine if a border was set. Borders also contain a middle section
 		// for tables, but those don't apply here, so we explicitly check for
 		// the relevant parts of borders only.
-		hasBorder = border.Top != "" ||
-			border.Bottom != "" ||
-			border.Left != "" ||
-			border.Right != "" ||
-			border.TopLeft != "" ||
-			border.TopRight != "" ||
-			border.BottomLeft != "" ||
-			border.BottomRight != ""
+		hasBorder = hasBorderTop ||
+			hasBorderBottom ||
+			hasBorderLeft ||
+			hasBorderRight ||
+			hasBorderTopLeft ||
+			hasBorderTopRight ||
+			hasBorderBottomLeft ||
+			hasBorderBottomRight
 
 		hasTop    = s.getAsBool(borderTopKey, false)
 		hasRight  = s.getAsBool(borderRightKey, false)
@@ -277,6 +289,39 @@ func (s Style) applyBorder(str string) string {
 		return str
 	}
 
+	// But should we really render the top border?
+	if !topSet && !hasBorderTop && !hasBorderTopLeft && !hasBorderTopRight {
+		hasTop = false
+	}
+
+	// And should we render the bottom border?
+	if !bottomSet && !hasBorderBottom && !hasBorderBottomLeft && !hasBorderBottomRight {
+		hasBottom = false
+	}
+
+	// Don't render horizontal borders if the top and bottom won't be rendered
+	// and the border edge isn't set.
+	//
+	// For example, we wouldn't render a left border in the following case
+	// because setting the right border only nullifies other borders:
+	//
+	//  Style().
+	//      BorderStyle(NormalBorder()).
+	//      BorderRight(true)
+	//
+	// We also wouldn't render the left border in the following case, where the
+	// top and bottom borders are missing and the left border doesn't have
+	// a value set:
+	//
+	//  Style().
+	//      BorderStyle(Border{Right: "│"})
+	if !hasBorderLeft && !hasTop && !hasBottom {
+		hasLeft = false
+	}
+	if !hasBorderRight && !hasTop && !hasBottom {
+		hasRight = false
+	}
+
 	lines, width := getLines(str)
 
 	if hasLeft {
@@ -290,8 +335,8 @@ func (s Style) applyBorder(str string) string {
 		border.Right = " "
 	}
 
-	// If corners should be rendered but are set with the empty string, fill them
-	// with a single space.
+	// If corners should be rendered but are set with the empty string, fill
+	// them with a single space.
 	if hasTop && hasLeft && border.TopLeft == "" {
 		border.TopLeft = " "
 	}
@@ -338,7 +383,7 @@ func (s Style) applyBorder(str string) string {
 
 	var out strings.Builder
 
-	// Render top
+	// Render top. This includes the top left and right corners.
 	if hasTop {
 		top := renderHorizontalEdge(border.TopLeft, border.Top, border.TopRight, width)
 		top = s.styleBorder(top, topFG, topBG)
@@ -352,7 +397,7 @@ func (s Style) applyBorder(str string) string {
 	rightRunes := []rune(border.Right)
 	rightIndex := 0
 
-	// Render sides
+	// Render sides. This never includes any corners.
 	for i, l := range lines {
 		if hasLeft {
 			r := string(leftRunes[leftIndex])
@@ -376,7 +421,7 @@ func (s Style) applyBorder(str string) string {
 		}
 	}
 
-	// Render bottom
+	// Render bottom. This includes the bottom left and right corners.
 	if hasBottom {
 		bottom := renderHorizontalEdge(border.BottomLeft, border.Bottom, border.BottomRight, width)
 		bottom = s.styleBorder(bottom, bottomFG, bottomBG)