Fix unwanted view adjustment after page down (#3555)

Fix regression introduced while implementing nano-like page up/down in
commit b2dbcb3e: if the view is already at the end of the buffer and
the last line is even above the bottom, i.e. there are some empty
lines displayed below the last line (e.g. if we have scrolled past the
last line via the mouse wheel), pressing PageDown not just moves the
cursor to the last line but also unexpectedly adjusts the view so that
the last line is exactly at the bottom.

internal/action/actions.go

@@ -46,6 +46,14 @@ func (h *BufPane) ScrollAdjust() {
 	h.SetView(v)
 }
 
+// ScrollReachedEnd returns true if the view is at the end of the buffer,
+// i.e. the last line of the buffer is in the view.
+func (h *BufPane) ScrollReachedEnd() bool {
+	v := h.GetView()
+	end := h.SLocFromLoc(h.Buf.End())
+	return h.Diff(v.StartLine, end) < h.BufView().Height
+}
+
 // MousePress is the event that should happen when a normal click happens
 // This is almost always bound to left click
 func (h *BufPane) MousePress(e *tcell.EventMouse) bool {
@@ -1707,7 +1715,7 @@ func (h *BufPane) SelectPageDown() bool {
 	}
 	h.MoveCursorDown(scrollAmount)
 	h.Cursor.SelectTo(h.Cursor.Loc)
-	if h.Cursor.Num == 0 {
+	if h.Cursor.Num == 0 && !h.ScrollReachedEnd() {
 		h.ScrollDown(scrollAmount)
 		h.ScrollAdjust()
 	}
@@ -1736,7 +1744,7 @@ func (h *BufPane) CursorPageDown() bool {
 	pageOverlap := int(h.Buf.Settings["pageoverlap"].(float64))
 	scrollAmount := h.BufView().Height - pageOverlap
 	h.MoveCursorDown(scrollAmount)
-	if h.Cursor.Num == 0 {
+	if h.Cursor.Num == 0 && !h.ScrollReachedEnd() {
 		h.ScrollDown(scrollAmount)
 		h.ScrollAdjust()
 	}