gui: test main panel goto-top/bottom helpers

Cover the gotoBottomOriginY origin math (scroll-past-bottom on/off, content
shorter than the view, empty content) and the isDoublePress timing used to
detect the vim-style "gg" double press.

pkg/gui/main_panel_test.go

@@ -0,0 +1,100 @@
+package gui
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGotoBottomOriginY(t *testing.T) {
+	type scenario struct {
+		name             string
+		totalLines       int
+		viewHeight       int
+		scrollPastBottom bool
+		expected         int
+	}
+
+	scenarios := []scenario{
+		{
+			name:             "keeps the last page on screen when not scrolling past bottom",
+			totalLines:       100,
+			viewHeight:       20,
+			scrollPastBottom: false,
+			expected:         80,
+		},
+		{
+			name:             "scrolls all content off the top when scrolling past bottom",
+			totalLines:       100,
+			viewHeight:       20,
+			scrollPastBottom: true,
+			expected:         100,
+		},
+		{
+			name:             "never returns a negative origin when content fits the view",
+			totalLines:       5,
+			viewHeight:       20,
+			scrollPastBottom: false,
+			expected:         0,
+		},
+		{
+			name:             "handles empty content",
+			totalLines:       0,
+			viewHeight:       20,
+			scrollPastBottom: true,
+			expected:         0,
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			assert.Equal(t, s.expected, gotoBottomOriginY(s.totalLines, s.viewHeight, s.scrollPastBottom))
+		})
+	}
+}
+
+func TestIsDoublePress(t *testing.T) {
+	now := time.Unix(1000, 0)
+	timeout := 250 * time.Millisecond
+
+	type scenario struct {
+		name     string
+		previous time.Time
+		now      time.Time
+		expected bool
+	}
+
+	scenarios := []scenario{
+		{
+			name:     "no previous press is not a double press",
+			previous: time.Time{},
+			now:      now,
+			expected: false,
+		},
+		{
+			name:     "second press within the timeout is a double press",
+			previous: now.Add(-100 * time.Millisecond),
+			now:      now,
+			expected: true,
+		},
+		{
+			name:     "second press exactly on the timeout is a double press",
+			previous: now.Add(-timeout),
+			now:      now,
+			expected: true,
+		},
+		{
+			name:     "second press after the timeout is not a double press",
+			previous: now.Add(-timeout - time.Millisecond),
+			now:      now,
+			expected: false,
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.name, func(t *testing.T) {
+			assert.Equal(t, s.expected, isDoublePress(s.previous, s.now, timeout))
+		})
+	}
+}