Make sure scrollbars have the right size initially

We refresh the view after reading just enough to fill it, so that we see the
initial content as quickly as possible, but then we continue reading enough
lines so that we can tell how long the scrollbar needs to be, and then we
refresh again. This can result in slight flicker of the scrollbar when it is
first drawn with a bigger size and then jumps to a smaller size; however, that's
a good tradeoff for a solution that provides both good speed and accuracy.

pkg/tasks/tasks.go

@@ -39,7 +39,7 @@ type ViewBufferManager struct {
 	taskIDMutex  deadlock.Mutex
 	Log          *logrus.Entry
 	newTaskID    int
-	readLines    chan int
+	readLines    chan LinesToRead
 	taskKey      string
 	onNewKey     func()
 
@@ -55,6 +55,16 @@ type ViewBufferManager struct {
 	throttle bool
 }
 
+type LinesToRead struct {
+	// Total number of lines to read
+	Total int
+
+	// Number of lines after which we have read enough to fill the view, and can
+	// do an initial refresh. Only set for the initial read request; -1 for
+	// subsequent requests.
+	InitialRefreshAfter int
+}
+
 func (m *ViewBufferManager) GetTaskKey() string {
 	return m.taskKey
 }
@@ -73,19 +83,19 @@ func NewViewBufferManager(
 		beforeStart:  beforeStart,
 		refreshView:  refreshView,
 		onEndOfInput: onEndOfInput,
-		readLines:    make(chan int, 1024),
+		readLines:    make(chan LinesToRead, 1024),
 		onNewKey:     onNewKey,
 	}
 }
 
 func (self *ViewBufferManager) ReadLines(n int) {
 	go utils.Safe(func() {
-		self.readLines <- n
+		self.readLines <- LinesToRead{Total: n, InitialRefreshAfter: -1}
 	})
 }
 
 // note: onDone may be called twice
-func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), prefix string, linesToRead int, onDone func()) func(chan struct{}) error {
+func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), prefix string, linesToRead LinesToRead, onDone func()) func(chan struct{}) error {
 	return func(stop chan struct{}) error {
 		var once sync.Once
 		var onDoneWrapper func()
@@ -130,7 +140,7 @@ func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), p
 		loadingMutex := deadlock.Mutex{}
 
 		// not sure if it's the right move to redefine this or not
-		self.readLines = make(chan int, 1024)
+		self.readLines = make(chan LinesToRead, 1024)
 
 		done := make(chan struct{})
 
@@ -163,7 +173,7 @@ func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), p
 				case <-stop:
 					break outer
 				case linesToRead := <-self.readLines:
-					for i := 0; i < linesToRead; i++ {
+					for i := 0; i < linesToRead.Total; i++ {
 						select {
 						case <-stop:
 							break outer
@@ -188,6 +198,13 @@ func (self *ViewBufferManager) NewCmdTask(start func() (*exec.Cmd, io.Reader), p
 							break outer
 						}
 						_, _ = self.writer.Write(append(scanner.Bytes(), '\n'))
+
+						if i+1 == linesToRead.InitialRefreshAfter {
+							// We have read enough lines to fill the view, so do a first refresh
+							// here to show what we have. Continue reading and refresh again at
+							// the end to make sure the scrollbar has the right size.
+							self.refreshView()
+						}
 					}
 					self.refreshView()
 				}