Lock Stack's mutex lock like with all other Controls.

stack.go

@@ -50,6 +50,9 @@ func (s *Stack) SetStretchy(index int) {
 }
 
 func (s *Stack) make(window *sysData) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
 	for i, c := range s.controls {
 		err := c.make(window)
 		if err != nil {
@@ -61,6 +64,9 @@ func (s *Stack) make(window *sysData) error {
 }
 
 func (s *Stack) setRect(x int, y int, width int, height int) error {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
 	var stretchywid, stretchyht int
 
 	if len(s.controls) == 0 {		// do nothing if there's nothing to do
@@ -121,6 +127,9 @@ func (s *Stack) setRect(x int, y int, width int, height int) error {
 
 // The preferred size of a Stack is the sum of the preferred sizes of non-stretchy controls + (the number of stretchy controls * the largest preferred size among all stretchy controls).
 func (s *Stack) preferredSize() (width int, height int, err error) {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
 	max := func(a int, b int) int {
 		if a > b {
 			return a

todo.md

@@ -25,7 +25,6 @@ so I don't forget:
 important things:
 - there's no GTK+ error handling whatsoever; we need to figure out how it works
 - make sure GTK+ documentation point differences don't matter
-- lock setRect() and preferredSize() in all Controls (including Stack)
 - button sizes and LineEdit sizes on Windows seem too big; Comboboxes have margins
 - sometimes the size of the drop-down part of a Combobox becomes 0 or 1 or some other impossibly small value on Windows