fix jiggle when scrolloff is maxed

eval.go

@@ -39,9 +39,8 @@ func (e *SetExpr) eval(app *App, args []string) {
 			log.Print(msg)
 			return
 		}
-		max := app.ui.wins[0].h/2 - 1
+		max := app.ui.wins[0].h/2
 		if n > max {
-			// TODO: stay at the same row while up/down in the middle
 			n = max
 		}
 		gOpts.scrolloff = n

nav.go

@@ -260,6 +260,11 @@ func (nav *Nav) down() {
 
 	dir.pos++
 	edge := min(gOpts.scrolloff, maxind-dir.ind)
+
+	// use a smaller value when the height is even and scrolloff is maxed
+	// in order to stay at the same row as much as possible while up/down
+	edge = min(edge, nav.height / 2 + nav.height % 2 - 1)
+
 	dir.pos = min(dir.pos, nav.height-edge-1)
 	dir.pos = min(dir.pos, maxind)
 }