confine scrolling to the contents of the box

1 files changed, 17 insertions, 6 deletions

diff --git a/tui/layout.go b/tui/layout.go
index 210924f..4feda2f 100644
--- a/tui/layout.go
+++ b/tui/layout.go
@@ -332,8 +332,19 @@ func (b *Box) computePositions(axis int) {
 			}
 		}
 		if b.Scroll != nil {
-			var first *Box
-			var last *Box
+			first := b.children[0]
+			last := b.children[len(b.children) - 1]
+
+			lastPos := last.computedPosition
+			lastPos += last.computedSize[axis]
+			if b.Dir.reverse() {
+				lastPos = b.computedSize[axis] - lastPos
+			}
+			b.Scroll.absolute = min(b.Scroll.absolute, lastPos - b.computedSize[axis])
+			b.Scroll.absolute = max(b.Scroll.absolute, 0)
+
+			var firstShown *Box
+			var lastShown *Box
 			for _, c := range b.children {
 				if !b.Dir.reverse() {
 					c.computedPosition -= b.Scroll.absolute
@@ -342,19 +353,19 @@ func (b *Box) computePositions(axis int) {
 				}
 				if c.computedPosition > b.computedRect.min[axis] &&
 						c.computedPosition < b.computedRect.max[axis] {
-					if first == nil {
+					if firstShown == nil {
 						first = c
 					}
 					last = c
 				}
 			}
-			if first == b.children[0] {
+			if firstShown == first {
 				b.Scroll.atFirst = true
 			}
-			if last == b.children[len(b.children) - 1] {
+			if lastShown == last {
 				b.Scroll.atLast = true
 			}
-			if first != nil {
+			if firstShown != nil {
 				p := first.computedPosition
 				if b.Dir.reverse() {
 					p = b.computedSize[axis] - p