Rework msglist scrolling

This changes the scrolling to be done on the draw, when the height is
updated, ensuring that the selected item is kept on screen during
resizing.

Also, this ensures that messages will fill the screen when resizing the
window, for instance, shrinking and then growing drags down more
messages if possible.

This is a transplant of the dirlist scrolling logic.

1 files changed, 30 insertions, 14 deletions

diff --git a/widgets/msglist.go b/widgets/msglist.go
index 5aedb446..38b6369a 100644
--- a/widgets/msglist.go
+++ b/widgets/msglist.go
@@ -63,6 +63,8 @@ func (ml *MessageList) Draw(ctx *ui.Context) {
 		}
 	}
 
+	ml.ensureScroll()
+
 	var (
 		needsHeaders []uint32
 		row          int = 0
@@ -179,12 +181,12 @@ func (ml *MessageList) MouseEvent(localX int, localY int, event tcell.Event) {
 			if ml.store != nil {
 				ml.store.Next()
 			}
-			ml.Scroll()
+			ml.Invalidate()
 		case tcell.WheelUp:
 			if ml.store != nil {
 				ml.store.Prev()
 			}
-			ml.Scroll()
+			ml.Invalidate()
 		}
 	}
 }
@@ -225,7 +227,6 @@ func (ml *MessageList) storeUpdate(store *lib.MessageStore) {
 		ml.nmsgs = len(uids)
 	}
 
-	ml.Scroll()
 	ml.Invalidate()
 }
 
@@ -266,25 +267,40 @@ func (ml *MessageList) Selected() *models.MessageInfo {
 func (ml *MessageList) Select(index int) {
 	store := ml.Store()
 	store.Select(index)
-	ml.Scroll()
+	ml.Invalidate()
 }
 
-func (ml *MessageList) Scroll() {
+func (ml *MessageList) ensureScroll() {
 	store := ml.Store()
-
 	if store == nil || len(store.Uids()) == 0 {
 		return
 	}
-	if ml.Height() != 0 {
-		// I'm too lazy to do the math right now
-		for store.SelectedIndex()-ml.scroll >= ml.Height() {
-			ml.scroll += 1
-		}
-		for store.SelectedIndex()-ml.scroll < 0 {
-			ml.scroll -= 1
+
+	h := ml.Height()
+
+	maxScroll := len(store.Uids()) - h
+	if maxScroll < 0 {
+		maxScroll = 0
+	}
+
+	selectedIndex := store.SelectedIndex()
+
+	if selectedIndex >= ml.scroll && selectedIndex < ml.scroll+h {
+		if ml.scroll > maxScroll {
+			ml.scroll = maxScroll
 		}
+		return
+	}
+
+	if selectedIndex >= ml.scroll+h {
+		ml.scroll = selectedIndex - h + 1
+	} else if selectedIndex < ml.scroll {
+		ml.scroll = selectedIndex
+	}
+
+	if ml.scroll > maxScroll {
+		ml.scroll = maxScroll
 	}
-	ml.Invalidate()
 }
 
 func (ml *MessageList) drawEmptyMessage(ctx *ui.Context) {