Move to top/bottom of page first, and only then scroll

Author: stefanhaller

pkg/gui/controllers/list_controller.go

@@ -139,23 +139,41 @@ func (self *ListController) handlePageChange(delta int) error {
 view := self.context.GetViewTrait()
 
 before := list.GetSelectedLineIdx()
-afterViewIdx := self.context.ModelIndexToViewIndex(before) + delta
-newModelIndex := self.context.ViewIndexToModelIndex(afterViewIdx)
-list.MoveSelectedLine(newModelIndex - before)
 
 viewPortStart, viewPortHeight := view.ViewPortYBounds()
+beforeViewIdx := self.context.ModelIndexToViewIndex(before)
+afterViewIdx := beforeViewIdx + delta
+newModelIndex := self.context.ViewIndexToModelIndex(afterViewIdx)
 
 if delta < 0 {
 // Previous page: keep selection at top of viewport
-linesToScroll := afterViewIdx - viewPortStart
-if linesToScroll < 0 {
-view.ScrollUp(-linesToScroll)
+indexAtTopOfPage := self.context.ViewIndexToModelIndex(viewPortStart)
+if before != indexAtTopOfPage {
+// If the selection isn't already at the top of the page, move it there without scrolling
+list.MoveSelectedLine(indexAtTopOfPage - before)
+} else {
+// Otherwise, move the selection by one page and scroll
+list.MoveSelectedLine(newModelIndex - before)
+
+linesToScroll := afterViewIdx - viewPortStart
+if linesToScroll < 0 {
+view.ScrollUp(-linesToScroll)
+}
 }
 } else {
 // Next page: keep selection at bottom of viewport
-linesToScroll := afterViewIdx - (viewPortStart + viewPortHeight - 1)
-if linesToScroll > 0 {
-view.ScrollDown(linesToScroll)
+indexAtBottomOfPage := self.context.ViewIndexToModelIndex(viewPortStart + viewPortHeight - 1)
+if before != indexAtBottomOfPage {
+// If the selection isn't already at the bottom of the page, move it there without scrolling
+list.MoveSelectedLine(indexAtBottomOfPage - before)
+} else {
+// Otherwise, move the selection by one page and scroll
+list.MoveSelectedLine(newModelIndex - before)
+
+linesToScroll := afterViewIdx - (viewPortStart + viewPortHeight - 1)
+if linesToScroll > 0 {
+view.ScrollDown(linesToScroll)
+}
 }
 }