scrollable: rewrite with scroll offset

Rewrite the scrolling logic to consider a scroll offset. Ensure correct
lower and upper bounds of the scroll variable. Cap offset at half of the
screen height.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Tested-by: Jason Cox <me@jasoncarloscox.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 25 insertions, 15 deletions

diff --git a/app/scrollable.go b/app/scrollable.go
index bca20232..3a0555fe 100644
--- a/app/scrollable.go
+++ b/app/scrollable.go
@@ -3,6 +3,7 @@ package app
 // Scrollable implements vertical scrolling
 type Scrollable struct {
 	scroll int
+	offset int
 	height int
 	elems  int
 }
@@ -11,6 +12,14 @@ func (s *Scrollable) Scroll() int {
 	return s.scroll
 }
 
+func (s *Scrollable) SetOffset(offset int) {
+	s.offset = offset
+}
+
+func (s *Scrollable) ScrollOffset() int {
+	return s.offset
+}
+
 func (s *Scrollable) PercentVisible() float64 {
 	if s.elems <= 0 {
 		return 1.0
@@ -38,30 +47,31 @@ func (s *Scrollable) UpdateScroller(height, elems int) {
 	s.elems = elems
 }
 
-func (s *Scrollable) EnsureScroll(selectingIdx int) {
-	if selectingIdx < 0 {
+func (s *Scrollable) EnsureScroll(idx int) {
+	if idx < 0 {
 		return
 	}
 
+	middle := s.height / 2
+	switch {
+	case s.offset > middle:
+		s.scroll = idx - middle
+	case idx < s.scroll+s.offset:
+		s.scroll = idx - s.offset
+	case idx >= s.scroll-s.offset+s.height:
+		s.scroll = idx + s.offset - s.height + 1
+	}
+
 	maxScroll := s.elems - s.height
 	if maxScroll < 0 {
 		maxScroll = 0
 	}
 
-	if selectingIdx >= s.scroll && selectingIdx < s.scroll+s.height {
-		if s.scroll > maxScroll {
-			s.scroll = maxScroll
-		}
-		return
-	}
-
-	if selectingIdx >= s.scroll+s.height {
-		s.scroll = selectingIdx - s.height + 1
-	} else if selectingIdx < s.scroll {
-		s.scroll = selectingIdx
-	}
-
 	if s.scroll > maxScroll {
 		s.scroll = maxScroll
 	}
+
+	if s.scroll < 0 {
+		s.scroll = 0
+	}
 }