update:scrollable list

internal/tui/scroll.go

@@ -0,0 +1,88 @@
+package tui
+
+// visibleRange returns [start, end) indices of rows that should be rendered.
+func visibleRange(total, selected, scrollOff, viewHeight int) (int, int) {
+	if total <= 0 {
+		return 0, 0
+	}
+
+	maxVis := viewHeight
+	if maxVis <= 0 {
+		maxVis = total // unconstrained
+	}
+	if maxVis >= total {
+		return 0, total
+	}
+
+	// Reserve lines for scroll indicators.
+	hasAbove := scrollOff > 0
+	hasBelow := scrollOff+maxVis < total
+	if hasAbove {
+		maxVis--
+	}
+	if hasBelow {
+		maxVis--
+	}
+	if maxVis < 1 {
+		maxVis = 1
+	}
+
+	start := scrollOff
+	end := start + maxVis
+
+	if selected < start {
+		start = selected
+		end = start + maxVis
+	}
+	if selected >= end {
+		end = selected + 1
+		start = end - maxVis
+	}
+	if start < 0 {
+		start = 0
+		end = min(maxVis, total)
+	}
+	if end > total {
+		end = total
+		start = max(0, end-maxVis)
+	}
+
+	return start, end
+}
+
+// syncScroll adjusts scrollOff so selected stays inside the viewport.
+func syncScroll(selected, total, viewHeight, scrollOff int) int {
+	if total <= 0 {
+		return 0
+	}
+	maxVis := viewHeight
+	if maxVis <= 0 || maxVis >= total {
+		return 0
+	}
+
+	if selected < 0 {
+		selected = 0
+	}
+	if selected >= total {
+		selected = total - 1
+	}
+
+	if scrollOff < 0 {
+		scrollOff = 0
+	}
+	if scrollOff > total-1 {
+		scrollOff = total - 1
+	}
+
+	if selected < scrollOff {
+		scrollOff = selected
+	}
+	if selected >= scrollOff+maxVis {
+		scrollOff = selected - maxVis + 1
+	}
+
+	if scrollOff < 0 {
+		scrollOff = 0
+	}
+	return scrollOff
+}