highlighter: Fix last open racy access in ReHighlightStates()

Otherwise it's possible that `ReHighlightStates()` called from the async
initial highlighting can be interrupted by the main goroutine, which then
changes the already set and furthermore used `h.lastRegion`. The resulting
last region respective set state isn't reliable any longer then.

pkg/highlight/highlighter.go

@@ -360,11 +360,11 @@ func (h *Highlighter) HighlightMatches(input LineStates, startline, endline int)
 func (h *Highlighter) ReHighlightStates(input LineStates, startline int) int {
 	// lines := input.LineData()
 
-	h.lastRegion = nil
+	var curState *region
 	if startline > 0 {
 		input.Lock()
 		if startline-1 < input.LinesNum() {
-			h.lastRegion = input.State(startline - 1)
+			curState = input.State(startline - 1)
 		}
 		input.Unlock()
 	}
@@ -379,12 +379,13 @@ func (h *Highlighter) ReHighlightStates(input LineStates, startline int) int {
 		// highlights := make(LineMatch)
 
 		// var match LineMatch
+		h.lastRegion = curState
 		if i == 0 || h.lastRegion == nil {
 			h.highlightEmptyRegion(nil, 0, true, i, line, true)
 		} else {
 			h.highlightRegion(nil, 0, true, i, line, h.lastRegion, true)
 		}
-		curState := h.lastRegion
+		curState = h.lastRegion
 		lastState := input.State(i)
 
 		input.SetState(i, curState)