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)