refactor page/list movement

Signed-off-by: Hadi <112569860+anotherhadi@users.noreply.github.com>

internal/util/cursor.go

@@ -0,0 +1,30 @@
+package util
+
+// CursorMovePage moves cursor forward or backward by one page (perPage items),
+// clamped to [0, total-1].
+func CursorMovePage(cursor, total, perPage int, forward bool) int {
+	step := perPage
+	if step < 1 {
+		step = 1
+	}
+	if forward {
+		cursor += step
+	} else {
+		cursor -= step
+	}
+	if cursor < 0 || total <= 0 {
+		return 0
+	}
+	if cursor >= total {
+		return total - 1
+	}
+	return cursor
+}
+
+// CursorGotoBottom returns the last valid cursor index for a list of total items.
+func CursorGotoBottom(total int) int {
+	if total <= 0 {
+		return 0
+	}
+	return total - 1
+}

internal/util/viewport.go

@@ -0,0 +1,39 @@
+package util
+
+import (
+	"charm.land/bubbles/v2/viewport"
+	tea "charm.land/bubbletea/v2"
+)
+
+// ScrollViewport scrolls vp vertically by half its height.
+// delta should be -1 for up, +1 for down.
+func ScrollViewport(vp *viewport.Model, delta int) {
+	step := vp.Height() / 2
+	if step < 1 {
+		step = 1
+	}
+	vp.SetYOffset(vp.YOffset() + delta*step)
+}
+
+// HandleMouseWheel applies standard mouse wheel scrolling to vp.
+// Vertical: one line at a time. Shift+vertical or horizontal: scroll 6 columns.
+func HandleMouseWheel(msg tea.MouseWheelMsg, vp *viewport.Model) {
+	switch msg.Button {
+	case tea.MouseWheelUp:
+		if msg.Mod.Contains(tea.ModShift) {
+			vp.ScrollLeft(6)
+		} else {
+			vp.SetYOffset(vp.YOffset() - 1)
+		}
+	case tea.MouseWheelDown:
+		if msg.Mod.Contains(tea.ModShift) {
+			vp.ScrollRight(6)
+		} else {
+			vp.SetYOffset(vp.YOffset() + 1)
+		}
+	case tea.MouseWheelLeft:
+		vp.ScrollLeft(6)
+	case tea.MouseWheelRight:
+		vp.ScrollRight(6)
+	}
+}