internal/ui: bug fix: replace initWindowMonitor usages with initMonitor

There were two similar member variables internalWindowMonitor and
initMonitor, and when SetMonitor is called, probably there were
some inconsitency between them.

This change fixes the issue by not using initWindowMonitor.

Closes #2779

internal/ui/ui_glfw.go

@@ -87,7 +87,6 @@ type userInterfaceImpl struct {
 	initFullscreen             bool
 	initCursorMode             CursorMode
 	initWindowDecorated        bool
-	initWindowMonitor          int
 	initWindowPositionXInDIP   int
 	initWindowPositionYInDIP   int
 	initWindowWidthInDIP       int
@@ -141,7 +140,6 @@ func init() {
 		maxWindowHeightInDIP:     glfw.DontCare,
 		initCursorMode:           CursorModeVisible,
 		initWindowDecorated:      true,
-		initWindowMonitor:        glfw.DontCare,
 		initWindowPositionXInDIP: invalidPos,
 		initWindowPositionYInDIP: invalidPos,
 		initWindowWidthInDIP:     640,
@@ -207,6 +205,9 @@ func initialize() error {
 }
 
 func (u *userInterfaceImpl) setInitMonitor(m *glfw.Monitor) {
+	u.m.Lock()
+	defer u.m.Unlock()
+
 	u.initMonitor = m
 	u.initDeviceScaleFactor = u.deviceScaleFactor(m)
 	// GetVideoMode must be called from the main thread, then call this here and record
@@ -426,24 +427,6 @@ func (u *userInterfaceImpl) setIconImages(iconImages []image.Image) {
 	u.m.Unlock()
 }
 
-func (u *userInterfaceImpl) getInitWindowMonitor() int {
-	u.m.RLock()
-	v := u.initWindowMonitor
-	u.m.RUnlock()
-	return v
-}
-
-func (u *userInterfaceImpl) setInitWindowMonitor(monitor int) {
-	if microsoftgdk.IsXbox() {
-		return
-	}
-
-	u.m.Lock()
-	defer u.m.Unlock()
-
-	u.initWindowMonitor = monitor
-}
-
 func (u *userInterfaceImpl) getInitWindowPositionInDIP() (int, int) {
 	if microsoftgdk.IsXbox() {
 		return 0, 0
@@ -797,7 +780,7 @@ func init() {
 // createWindow must be called from the main thread.
 //
 // createWindow does not set the position or size so far.
-func (u *userInterfaceImpl) createWindow(width, height int, monitor int) error {
+func (u *userInterfaceImpl) createWindow(width, height int, monitor *glfw.Monitor) error {
 	if u.window != nil {
 		panic("ui: u.window must not exist at createWindow")
 	}
@@ -809,12 +792,10 @@ func (u *userInterfaceImpl) createWindow(width, height int, monitor int) error {
 	}
 
 	// Set our target monitor if provided. This is required to prevent an initial window flash on the default monitor.
-	if monitor != glfw.DontCare {
+	x, y := monitor.GetPos()
-		m := theMonitors.monitorFromID(monitor)
+	px, py := InitialWindowPosition(monitor.GetVideoMode().Width, monitor.GetVideoMode().Height, width, height)
-		x, y := m.m.GetPos()
-		px, py := InitialWindowPosition(m.m.GetVideoMode().Width, m.m.GetVideoMode().Height, width, height)
 	window.SetPos(x+px, y+py)
-	}
+
 	initializeWindowAfterCreation(window)
 
 	u.window = window
@@ -1008,23 +989,16 @@ func (u *userInterfaceImpl) initOnMainThread(options *RunOptions) error {
 		glfw.WindowHint(glfw.Visible, glfw.True)
 	}
 
-	// Get our target monitor.
-	monitor := u.getInitWindowMonitor()
-
-	if monitor != glfw.DontCare {
-		u.setInitMonitor(theMonitors.monitorFromID(monitor).m)
-	}
-
 	ww, wh := u.getInitWindowSizeInDIP()
 	initW := int(u.dipToGLFWPixel(float64(ww), u.initMonitor))
 	initH := int(u.dipToGLFWPixel(float64(wh), u.initMonitor))
-	if err := u.createWindow(initW, initH, monitor); err != nil {
+	if err := u.createWindow(initW, initH, u.initMonitor); err != nil {
 		return err
 	}
 
 	// The position must be set before the size is set (#1982).
 	// setWindowSize refers the current monitor's device scale.
-	// TODO: currentMonitor is very hard to use correctly. Refactor this.
+	// TODO: The window position is already set at createWindow. Unify the logic.
 	wx, wy := u.getInitWindowPositionInDIP()
 	// Force to put the window in the initial monitor (#1575).
 	if wx < 0 {

internal/ui/window_glfw.go

@@ -234,7 +234,7 @@ func (w *glfwWindow) SetMonitor(monitor *Monitor) {
 		return
 	}
 	if !w.ui.isRunning() {
-		w.ui.setInitWindowMonitor(monitor.id)
+		w.ui.setInitMonitor(monitor.m)
 		return
 	}
 	w.ui.mainThread.Call(func() {