From 07480ed66d6bf08af5659a219642d0c3b29cb6e4 Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Sun, 23 Aug 2020 23:37:58 +0900
Subject: [PATCH] uidriver/glfw: Add better fallbacks for
 currentMonitorFromPosition

Updates #1119
---
 internal/uidriver/glfw/ui.go         | 11 ++++++-----
 internal/uidriver/glfw/ui_darwin.go  | 18 ++++++++++--------
 internal/uidriver/glfw/ui_windows.go | 16 +++++++++++-----
 3 files changed, 27 insertions(+), 18 deletions(-)

diff --git a/internal/uidriver/glfw/ui.go b/internal/uidriver/glfw/ui.go
index 734807076..b29fa8937 100644
--- a/internal/uidriver/glfw/ui.go
+++ b/internal/uidriver/glfw/ui.go
@@ -1094,12 +1094,13 @@ func (u *UserInterface) updateVsync() {
 //
 // currentMonitor must be called on the main thread.
 func (u *UserInterface) currentMonitor() *glfw.Monitor {
-	if w := u.window; w != nil {
-		// GetMonitor is available only on fullscreen.
-		if m := w.GetMonitor(); m != nil {
-			return m
-		}
+	// GetMonitor is available only on fullscreen.
+	if m := u.window.GetMonitor(); m != nil {
+		return m
 	}
+
+	// Getting a monitor from a window position is not reliable in general (e.g., when a window is put across
+	// multiple monitors).
 	// Get the monitor which the current window belongs to. This requires OS API.
 	return u.currentMonitorFromPosition()
 }
diff --git a/internal/uidriver/glfw/ui_darwin.go b/internal/uidriver/glfw/ui_darwin.go
index 89da75b69..eb1f3ae43 100644
--- a/internal/uidriver/glfw/ui_darwin.go
+++ b/internal/uidriver/glfw/ui_darwin.go
@@ -59,16 +59,18 @@ func (u *UserInterface) currentMonitorFromPosition() *glfw.Monitor {
 	x := C.int(0)
 	y := C.int(0)
 	// Note: [NSApp mainWindow] is nil when it doesn't have its border. Use u.window here.
-	if u.window != nil {
-		win := u.window.GetCocoaWindow()
-		C.currentMonitorPos(win, &x, &y)
-		for _, m := range glfw.GetMonitors() {
-			mx, my := m.GetPos()
-			if int(x) == mx && int(y) == my {
-				return m
-			}
+	win := u.window.GetCocoaWindow()
+	C.currentMonitorPos(win, &x, &y)
+	for _, m := range glfw.GetMonitors() {
+		mx, my := m.GetPos()
+		if int(x) == mx && int(y) == my {
+			return m
 		}
 	}
+
+	if m, ok := getCachedMonitor(u.window.GetPos()); ok {
+		return m.m
+	}
 	return glfw.GetPrimaryMonitor()
 }
 
diff --git a/internal/uidriver/glfw/ui_windows.go b/internal/uidriver/glfw/ui_windows.go
index 17c7a5bb7..cf05e70fb 100644
--- a/internal/uidriver/glfw/ui_windows.go
+++ b/internal/uidriver/glfw/ui_windows.go
@@ -121,6 +121,13 @@ func (u *UserInterface) adjustWindowPosition(x, y int) (int, int) {
 }
 
 func (u *UserInterface) currentMonitorFromPosition() *glfw.Monitor {
+	fallback := func() *glfw.Monitor {
+		if m, ok := getCachedMonitor(u.window.GetPos()); ok {
+			return m.m
+		}
+		return glfw.GetPrimaryMonitor()
+	}
+
 	// TODO: Should we use u.window.GetWin32Window() here?
 	w, err := getActiveWindow()
 	if err != nil {
@@ -134,9 +141,8 @@ func (u *UserInterface) currentMonitorFromPosition() *glfw.Monitor {
 			panic(err)
 		}
 		if w == 0 {
-			// GetForegroundWindow can return null according to the document. Use
-			// the primary monitor instead.
-			return glfw.GetPrimaryMonitor()
+			// GetForegroundWindow can return null according to the document.
+			return fallback()
 		}
 	}
 
@@ -146,7 +152,7 @@ func (u *UserInterface) currentMonitorFromPosition() *glfw.Monitor {
 	m, err := monitorFromWindow(w, monitorDefaultToNearest)
 	if err != nil {
 		// monitorFromWindow can return error on Wine. Ignore this.
-		return glfw.GetPrimaryMonitor()
+		return fallback()
 	}
 
 	mi := monitorInfo{}
@@ -162,7 +168,7 @@ func (u *UserInterface) currentMonitorFromPosition() *glfw.Monitor {
 			return m
 		}
 	}
-	return glfw.GetPrimaryMonitor()
+	return fallback()
 }
 
 func (u *UserInterface) nativeWindow() unsafe.Pointer {