Rerender visible lines when scrolling by page

This fixes a bug in ListContextTrait.FocusLine whereby the view would go blank
when scrolling by page (using ',' or '.') in views that have
renderOnlyVisibleLines set to true but refreshViewportOnChange set to false.
Currently we don't have any such views; the only ones who use
renderOnlyVisibleLines are commits and subcommits, and they also use
refreshViewportOnChange. However, we are going to add one in the next commit,
and eventually it might be a good idea to convert all our list views to that by
default, and get rid of the renderOnlyVisibleLines flag.

pkg/gui/context/list_context_trait.go

@@ -21,6 +21,9 @@ type ListContextTrait struct {
 	// If this is true, we only render the visible lines of the list. Useful for lists that can
 	// get very long, because it can save a lot of memory
 	renderOnlyVisibleLines bool
+	// If renderOnlyVisibleLines is true, needRerenderVisibleLines indicates whether we need to
+	// rerender the visible lines e.g. because the scroll position changed
+	needRerenderVisibleLines bool
 }
 
 func (self *ListContextTrait) IsListContext() {}
@@ -50,8 +53,8 @@ func (self *ListContextTrait) FocusLine(scrollIntoView bool) {
 			self.refreshViewport()
 		} else if self.renderOnlyVisibleLines {
 			newOrigin, _ := self.GetViewTrait().ViewPortYBounds()
-			if oldOrigin != newOrigin {
-				self.HandleRender()
+			if oldOrigin != newOrigin || self.needRerenderVisibleLines {
+				self.refreshViewport()
 			}
 		}
 		return nil
@@ -105,6 +108,7 @@ func (self *ListContextTrait) HandleRender() {
 		startIdx, length := self.GetViewTrait().ViewPortYBounds()
 		content := self.renderLines(startIdx, startIdx+length)
 		self.GetViewTrait().SetViewPortContentAndClearEverythingElse(totalLength, content)
+		self.needRerenderVisibleLines = false
 	} else {
 		content := self.renderLines(-1, -1)
 		self.GetViewTrait().SetContent(content)
@@ -141,6 +145,10 @@ func (self *ListContextTrait) RenderOnlyVisibleLines() bool {
 	return self.renderOnlyVisibleLines
 }
 
+func (self *ListContextTrait) SetNeedRerenderVisibleLines() {
+	self.needRerenderVisibleLines = true
+}
+
 func (self *ListContextTrait) TotalContentHeight() int {
 	result := self.list.Len()
 	if self.getNonModelItems != nil {

pkg/gui/controllers/list_controller.go

@@ -178,6 +178,13 @@ func (self *ListController) handlePageChange(delta int) error {
 		}
 	}
 
+	// Since we already scrolled the view above, the normal mechanism that
+	// ListContextTrait.FocusLine uses for deciding whether rerendering is needed won't work. It is
+	// based on checking whether the origin was changed by the call to FocusPoint in that function,
+	// but since we scrolled the view directly above, the origin has already been updated. So we
+	// must tell it explicitly to rerender.
+	self.context.SetNeedRerenderVisibleLines()
+
 	// Since we are maintaining the scroll position ourselves above, there's no point in passing
 	// ScrollSelectionIntoView=true here.
 	self.context.HandleFocus(types.OnFocusOpts{})

pkg/gui/types/context.go

@@ -182,6 +182,7 @@ type IListContext interface {
 	IsListContext() // used for type switch
 	RangeSelectEnabled() bool
 	RenderOnlyVisibleLines() bool
+	SetNeedRerenderVisibleLines()
 
 	IndexForGotoBottom() int
 }