internal/ui: refactoring: (*monitors).update must be called from the main thread

In the current implementation, (*monitors).update is not called
from other threads, but the current code is fragile.

internal/ui/monitor_glfw.go

@@ -63,7 +63,7 @@ var theMonitors monitors
 
 func (m *monitors) append(ms []*Monitor) []*Monitor {
 	if atomic.LoadInt32(&m.updateCalled) == 0 {
-		m.update()
+		panic("ui: (*monitors).update must be called before (*monitors).append is called")
 	}
 
 	m.m.Lock()
@@ -91,6 +91,7 @@ func (m *monitors) monitorFromID(id int) *Monitor {
 	return m.monitors[id]
 }
 
+// update must be called from the main thread.
 func (m *monitors) update() {
 	glfwMonitors := glfw.GetMonitors()
 	newMonitors := make([]*Monitor, 0, len(glfwMonitors))

internal/ui/ui_glfw.go

@@ -155,13 +155,13 @@ func init() {
 
 func init() {
 	hideConsoleWindowOnWindows()
+
 	if err := initialize(); err != nil {
 		panic(err)
 	}
 	glfw.SetMonitorCallback(glfw.ToMonitorCallback(func(monitor *glfw.Monitor, event glfw.PeripheralEvent) {
 		theMonitors.update()
 	}))
-	theMonitors.update()
 }
 
 var glfwSystemCursors = map[CursorShape]*glfw.Cursor{}
@@ -174,6 +174,9 @@ func initialize() error {
 	glfw.WindowHint(glfw.Visible, glfw.False)
 	glfw.WindowHint(glfw.ClientAPI, glfw.NoAPI)
 
+	// Update the monitor first. The monitor state is depended on various functions like initialMonitorByOS.
+	theMonitors.update()
+
 	m, err := initialMonitorByOS()
 	if err != nil {
 		return err