diff --git a/app/os_macos.go b/app/os_macos.go
index bf32184b..62426a91 100644
--- a/app/os_macos.go
+++ b/app/os_macos.go
@@ -226,7 +226,7 @@ func (w *window) Configure(options []Option) {
 	cnf.MinSize = cnf.MinSize.Div(int(screenScale))
 	cnf.MaxSize = cnf.MaxSize.Div(int(screenScale))
 
-	if prev.Size != cnf.Size {
+	if cnf.Mode != Fullscreen && prev.Size != cnf.Size {
 		w.config.Size = cnf.Size
 		C.setSize(w.window, C.CGFloat(cnf.Size.X), C.CGFloat(cnf.Size.Y))
 	}
diff --git a/app/os_windows.go b/app/os_windows.go
index 7cd62913..59a9a98f 100644
--- a/app/os_windows.go
+++ b/app/os_windows.go
@@ -496,7 +496,7 @@ func (w *window) Configure(options []Option) {
 	cnf := w.config
 	cnf.apply(cfg, options)
 
-	if prev.Size != cnf.Size {
+	if cnf.Mode != Fullscreen && prev.Size != cnf.Size {
 		width := int32(cnf.Size.X)
 		height := int32(cnf.Size.Y)
 		w.config.Size = cnf.Size
diff --git a/app/os_x11.go b/app/os_x11.go
index 67e58348..f2cd3133 100644
--- a/app/os_x11.go
+++ b/app/os_x11.go
@@ -137,7 +137,7 @@ func (w *x11Window) Configure(options []Option) {
 		C.XSetWMNormalHints(w.x, w.xw, &shints)
 	}
 
-	if prev.Size != cnf.Size {
+	if cnf.Mode != Fullscreen && prev.Size != cnf.Size {
 		w.config.Size = cnf.Size
 		C.XResizeWindow(w.x, w.xw, C.uint(cnf.Size.X), C.uint(cnf.Size.Y))
 	}
diff --git a/app/window.go b/app/window.go
index dd44cc60..701a0f53 100644
--- a/app/window.go
+++ b/app/window.go
@@ -625,7 +625,8 @@ func Title(t string) Option {
 	}
 }
 
-// Size sets the size of the window.
+// Size sets the size of the window. The option is ignored
+// in Fullscreen mode.
 func Size(w, h unit.Value) Option {
 	if w.V <= 0 {
 		panic("width must be larger than or equal to 0")