ui: Bug fix: The window was not shown on the secondary monitor on launching

This change changes the behavior of WindowPosition /
SetWindowPosition. The window position is now a relative position
and the origin position is the left-upper of the current monitor.

Fixes #1115

examples/mascot/main.go

@@ -87,7 +87,6 @@ func (m *mascot) update(screen *ebiten.Image) error {
 	m.count++
 
 	sw, sh := ebiten.ScreenSizeInFullscreen()
-	// TODO: Consider multiple monitors. This requires new APIs (#1114).
 	ebiten.SetWindowPosition(m.x16/16, m.y16/16+sh-height)
 
 	if m.vx16 == 0 {

internal/driver/ui.go

@@ -42,7 +42,6 @@ type UI interface {
 	IsVsyncEnabled() bool
 	ScreenSizeInFullscreen() (int, int)
 	IsScreenTransparent() bool
-	MonitorPosition() (int, int)
 
 	SetCursorMode(mode CursorMode)
 	SetFullscreen(fullscreen bool)

internal/uidriver/glfw/ui.go

@@ -1043,23 +1043,6 @@ func (u *UserInterface) IsScreenTransparent() bool {
 	return val
 }
 
-func (u *UserInterface) MonitorPosition() (int, int) {
-	if !u.isRunning() {
-		return u.monitorPosition()
-	}
-	var mx, my int
-	_ = u.t.Call(func() error {
-		mx, my = u.monitorPosition()
-		return nil
-	})
-	return mx, my
-}
-
-func (u *UserInterface) monitorPosition() (int, int) {
-	// TODO: toDeviceIndependentPixel might be required.
-	return u.currentMonitor().GetPos()
-}
-
 func (u *UserInterface) Input() driver.Input {
 	return &u.input
 }

internal/uidriver/glfw/window.go

@@ -185,6 +185,9 @@ func (w *window) Position() (int, int) {
 		} else {
 			wx, wy = w.ui.window.GetPos()
 		}
+		mx, my := w.ui.currentMonitor().GetPos()
+		wx -= mx
+		wy -= my
 		xf := w.ui.toDeviceIndependentPixel(float64(wx))
 		yf := w.ui.toDeviceIndependentPixel(float64(wy))
 		x, y = int(xf), int(yf)
@@ -199,9 +202,10 @@ func (w *window) SetPosition(x, y int) {
 		return
 	}
 	_ = w.ui.t.Call(func() error {
+		mx, my := w.ui.currentMonitor().GetPos()
 		xf := w.ui.toDeviceDependentPixel(float64(x))
 		yf := w.ui.toDeviceDependentPixel(float64(y))
-		x, y := adjustWindowPosition(int(xf), int(yf))
+		x, y := adjustWindowPosition(mx+int(xf), my+int(yf))
 		if w.ui.isFullscreen() {
 			w.ui.origPosX, w.ui.origPosY = x, y
 		} else {

internal/uidriver/js/ui.go

@@ -456,10 +456,6 @@ func (u *UserInterface) IsScreenTransparent() bool {
 	return bodyStyle.Get("backgroundColor").String() == "transparent"
 }
 
-func (u *UserInterface) MonitorPosition() (int, int) {
-	return 0, 0
-}
-
 func (u *UserInterface) Input() driver.Input {
 	return &u.input
 }

internal/uidriver/mobile/ui.go

@@ -433,10 +433,6 @@ func (u *UserInterface) IsScreenTransparent() bool {
 	return false
 }
 
-func (u *UserInterface) MonitorPosition() (int, int) {
-	return 0, 0
-}
-
 func (u *UserInterface) Input() driver.Input {
 	return &u.input
 }

window.go

@@ -112,6 +112,8 @@ func SetWindowIcon(iconImages []image.Image) {
 }
 
 // WindowPosition returns the window position.
+// The origin position is the left-upper corner of the current monitor.
+// The unit is device-independent pixels.
 //
 // WindowPosition panics if the main loop does not start yet.
 //
@@ -131,6 +133,8 @@ func WindowPosition() (x, y int) {
 }
 
 // SetWindowPosition sets the window position.
+// The origin position is the left-upper corner of the current monitor.
+// The unit is device-independent pixels.
 //
 // SetWindowPosition does nothing on fullscreen mode.
 //
@@ -194,10 +198,9 @@ func fixWindowPosition(width, height int) {
 	}
 
 	if initWindowPosition.x == invalidPos || initWindowPosition.y == invalidPos {
-		mx, my := uiDriver().MonitorPosition()
 		sw, sh := uiDriver().ScreenSizeInFullscreen()
-		x := mx + (sw-width)/2
+		x := (sw - width) / 2
-		y := my + (sh-height)/3
+		y := (sh - height) / 3
 		w.SetPosition(x, y)
 	} else {
 		w.SetPosition(initWindowPosition.x, initWindowPosition.y)