app: process one event at a time

Commit 11aec807b2 added a waiting flag to avoid
processing platform events recursively. However the flag was only true when the
window goroutine is blocked waiting for client events, so deferred window
functions such as Window.Option may still runs multiple times for recursive
events.

This change renames the flag to busy and sets it during the entire processing
of an event, including recursive events and deferred window functions.

Fixes: https://todo.sr.ht/~eliasnaur/gio/344
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/window.go

@@ -46,12 +46,10 @@ type Window struct {
 	// scheduledRedraws is sent the most recent delayed redraw time.
 	scheduledRedraws chan time.Time
 
-	waiting    bool
-	waitEvents []event.Event
-	defers     []func(d driver)
-	out        chan event.Event
-	frames     chan *op.Ops
-	frameAck   chan struct{}
+	defers   []func(d driver)
+	out      chan event.Event
+	frames   chan *op.Ops
+	frameAck chan struct{}
 	// dead is closed when the window is destroyed.
 	dead chan struct{}
 
@@ -81,8 +79,10 @@ type Window struct {
 }
 
 type callbacks struct {
-	w *Window
-	d driver
+	w          *Window
+	d          driver
+	busy       bool
+	waitEvents []event.Event
 }
 
 // queue is an event.Queue implementation that distributes system events
@@ -398,21 +398,23 @@ func (c *callbacks) Event(e event.Event) {
 	if c.d == nil {
 		panic("event while no driver active")
 	}
-	if c.w.waiting {
-		c.w.waitEvents = append(c.w.waitEvents, e)
+	if c.busy {
+		c.waitEvents = append(c.waitEvents, e)
 		return
 	}
+	c.busy = true
+	defer func() {
+		c.busy = false
+	}()
 	c.w.processEvent(c.d, e)
-	for len(c.w.waitEvents) > 0 {
-		we := c.w.waitEvents[0]
-		copy(c.w.waitEvents, c.w.waitEvents[1:])
-		c.w.waitEvents = c.w.waitEvents[:len(c.w.waitEvents)-1]
-		c.w.processEvent(c.d, we)
+	for _, e := range c.waitEvents {
+		c.w.processEvent(c.d, e)
 	}
+	c.waitEvents = c.waitEvents[:0]
 	for _, f := range c.w.defers {
 		f(c.d)
 	}
-	c.w.defers = nil
+	c.w.defers = c.w.defers[:0]
 	c.w.updateState(c.d)
 }
 
@@ -443,10 +445,6 @@ func (c *callbacks) SemanticAt(pos f32.Point) (router.SemanticID, bool) {
 }
 
 func (w *Window) waitAck() {
-	w.waiting = true
-	defer func() {
-		w.waiting = false
-	}()
 	for {
 		select {
 		case f := <-w.driverFuncs:
@@ -478,10 +476,6 @@ func (w *Window) destroyGPU() {
 // or to continue event handling. It returns whether the client
 // called Frame or not.
 func (w *Window) waitFrame() (*op.Ops, bool) {
-	w.waiting = true
-	defer func() {
-		w.waiting = false
-	}()
 	for {
 		select {
 		case f := <-w.driverFuncs: