Quick changes: turned "Window has been opened" to "Window has been created" since I'm going to separate creating and opening (Open() will call Create()).

button.go

@@ -9,7 +9,7 @@ import (
 // A Button represents a clickable button with some text.
 type Button struct {
 	// Clicked gets a message when the button is clicked.
-	// You cannot change it once the Window containing the Button has been opened.
+	// You cannot change it once the Window containing the Button has been created.
 	// If you do not respond to this signal, nothing will happen.
 	Clicked	chan struct{}
 

stack.go

@@ -48,7 +48,7 @@ func NewVerticalStack(controls ...Control) *Stack {
 	return newStack(vertical, controls...)
 }
 
-// SetStretchy marks a control in a Stack as stretchy. This cannot be called once the Window containing the Stack has been opened.
+// SetStretchy marks a control in a Stack as stretchy. This cannot be called once the Window containing the Stack has been created.
 // It panics if index is out of range.
 func (s *Stack) SetStretchy(index int) {
 	s.lock.Lock()

window.go

@@ -10,7 +10,7 @@ import (
 // Window represents an on-screen window.
 type Window struct {
 	// Closing gets a message when the user clicks the window's close button.
-	// You cannot change it once the Window has been opened.
+	// You cannot change it once the Window has been created.
 	// If you do not respond to this signal, nothing will happen; regardless of whether you handle the signal or not, the window will not be closed.
 	Closing		chan struct{}