Consider Viewport Frame Size when Calculating Visible Lines

viewport/viewport.go

@@ -111,18 +111,20 @@ func (m *Model) SetContent(s string) {
 // maxYOffset returns the maximum possible value of the y-offset based on the
 // viewport's content and set height.
 func (m Model) maxYOffset() int {
-	return max(0, len(m.lines)-m.Height)
+	height := m.Height - m.Style.GetVerticalFrameSize()
+	return clamp(len(m.lines)-height, 0, len(m.lines)-1)
 }
 
 // visibleLines returns the lines that should currently be visible in the
 // viewport.
-func (m Model) visibleLines() (lines []string) {
-	if len(m.lines) > 0 {
-		top := max(0, m.YOffset)
-		bottom := clamp(m.YOffset+m.Height, top, len(m.lines))
-		lines = m.lines[top:bottom]
+func (m Model) visibleLines() []string {
+	verticalSpace := m.Height - m.Style.GetVerticalFrameSize()
+	if verticalSpace <= 0 || len(m.lines) <= 0 {
+		return []string{}
 	}
-	return lines
+	top := clamp(m.YOffset, 0, len(m.lines)-1)
+	bottom := min(top+verticalSpace, len(m.lines))
+	return m.lines[top:bottom]
 }
 
 // scrollArea returns the scrollable boundaries for high performance rendering.

viewport/viewport_test.go

@@ -0,0 +1,213 @@
+package viewport
+
+import (
+	"strings"
+	"testing"
+	"unicode"
+
+	"github.com/MakeNowJust/heredoc"
+	"github.com/acarl005/stripansi"
+	"github.com/charmbracelet/lipgloss"
+)
+
+const (
+	viewportH = 8
+	viewportW = 15
+	content   = "line 1\nline 2\nline 3\nline 4\nline 5\nline 6\nline 7\nline 8\nline 9\nline 10"
+)
+
+func TestMaxYOffset(t *testing.T) {
+	type want struct {
+		maxYOffset    int
+		scrollTopView string
+		scrollBotView string
+	}
+
+	tests := []struct {
+		name  string
+		style lipgloss.Style
+		want  want
+	}{
+		{
+			name:  "no style",
+			style: lipgloss.NewStyle(),
+			want: want{
+				maxYOffset: 2,
+				scrollTopView: heredoc.Doc(`
+					line 1
+					line 2
+					line 3
+					line 4
+					line 5
+					line 6
+					line 7
+					line 8
+				`),
+				scrollBotView: heredoc.Doc(`
+					line 3
+					line 4
+					line 5
+					line 6
+					line 7
+					line 8
+					line 9
+					line 10
+				`),
+			},
+		},
+		{
+			name:  "with single border",
+			style: lipgloss.NewStyle().Border(lipgloss.RoundedBorder()),
+			want: want{
+				maxYOffset: 4,
+				scrollTopView: heredoc.Doc(`
+					╭─────────────╮
+					│line 1       │
+					│line 2       │
+					│line 3       │
+					│line 4       │
+					│line 5       │
+					│line 6       │
+					╰─────────────╯
+				`),
+				scrollBotView: heredoc.Doc(`
+					╭─────────────╮
+					│line 5       │
+					│line 6       │
+					│line 7       │
+					│line 8       │
+					│line 9       │
+					│line 10      │
+					╰─────────────╯
+				`),
+			},
+		},
+		{
+			name:  "with border + padding",
+			style: lipgloss.NewStyle().Border(lipgloss.RoundedBorder()).Padding(1, 1),
+			want: want{
+				maxYOffset: 6,
+				scrollTopView: heredoc.Doc(`
+					╭─────────────╮
+					│             │
+					│ line 1      │
+					│ line 2      │
+					│ line 3      │
+					│ line 4      │
+					│             │
+					╰─────────────╯
+				`),
+				scrollBotView: heredoc.Doc(`
+					╭─────────────╮
+					│             │
+					│ line 7      │
+					│ line 8      │
+					│ line 9      │
+					│ line 10     │
+					│             │
+					╰─────────────╯
+				`),
+			},
+		},
+		{
+			name:  "with border + margin",
+			style: lipgloss.NewStyle().Border(lipgloss.RoundedBorder()).Margin(1, 0),
+			want: want{
+				maxYOffset: 6,
+				scrollTopView: heredoc.Doc(`
+
+					╭─────────────╮
+					│line 1       │
+					│line 2       │
+					│line 3       │
+					│line 4       │
+					╰─────────────╯
+
+				`),
+				scrollBotView: heredoc.Doc(`
+
+					╭─────────────╮ 
+					│line 7       │ 
+					│line 8       │ 
+					│line 9       │ 
+					│line 10      │ 
+					╰─────────────╯
+
+				`),
+			},
+		},
+		{
+			name:  "with border + margin + padding",
+			style: lipgloss.NewStyle().Border(lipgloss.RoundedBorder()).Margin(1, 0).Padding(1, 1),
+			want: want{
+				maxYOffset: 8,
+				scrollTopView: heredoc.Doc(`
+
+					╭─────────────╮
+					│             │
+					│ line 1      │
+					│ line 2      │
+					│             │
+					╰─────────────╯
+
+				`),
+				scrollBotView: heredoc.Doc(`
+
+					╭─────────────╮
+					│             │
+					│ line 9      │
+					│ line 10     │
+					│             │
+					╰─────────────╯
+
+				`),
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			viewport := New(viewportW, viewportH)
+			viewport.Style = tt.style.Copy()
+			viewport.SetContent(content)
+
+			viewport, _ = viewport.Update(nil)
+
+			maxYOffset := viewport.maxYOffset()
+			if maxYOffset != tt.want.maxYOffset {
+				t.Fatalf("\nWant maxYOffset:\n%v\nGot:\n%v\n", tt.want.maxYOffset, maxYOffset)
+			}
+
+			viewport.SetYOffset(0)
+			viewTop := stripString(viewport.View())
+			wantViewTop := stripString(tt.want.scrollTopView)
+
+			if viewTop != wantViewTop {
+				t.Fatalf("Want view (when scrolled to top):\n%v\nGot:\n%v\n", wantViewTop, viewTop)
+			}
+
+			viewport.SetYOffset(100)
+			viewBot := stripString(viewport.View())
+			wantViewBot := stripString(tt.want.scrollBotView)
+
+			if viewBot != wantViewBot {
+				t.Fatalf("Want view (when scrolled to bottom):\n%v\nGot:\n%v\n", wantViewBot, viewBot)
+			}
+		})
+	}
+}
+
+func stripString(str string) string {
+	s := stripansi.Strip(str)
+	ss := strings.Split(s, "\n")
+
+	var lines []string
+	for _, l := range ss {
+		trim := strings.TrimRightFunc(l, unicode.IsSpace)
+		if trim != "" {
+			lines = append(lines, trim)
+		}
+	}
+
+	return strings.Join(lines, "\n")
+}