diff --git a/src/posix/cocoa/i_video.mm b/src/posix/cocoa/i_video.mm
index 8cc8fe0183..cb54f76c92 100644
--- a/src/posix/cocoa/i_video.mm
+++ b/src/posix/cocoa/i_video.mm
@@ -355,6 +355,9 @@ SystemGLFrameBuffer::SystemGLFrameBuffer(void*, const bool fullscreen)
 	assert(frameBuffer == nullptr);
 	frameBuffer = this;
 
+	// To be able to use OpenGL functions in SetMode()
+	ogl_LoadFunctions();
+
 	FConsoleWindow::GetInstance().Show(false);
 }
 
@@ -527,6 +530,14 @@ void SystemGLFrameBuffer::SetMode(const bool fullscreen, const bool hiDPI)
 		SetWindowedMode();
 	}
 
+	const NSSize viewSize = I_GetContentViewSize(m_window);
+
+	glViewport(0, 0, static_cast<GLsizei>(viewSize.width), static_cast<GLsizei>(viewSize.height));
+	glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
+	glClear(GL_COLOR_BUFFER_BIT);
+
+	[[NSOpenGLContext currentContext] flushBuffer];
+
 	[m_window updateTitle];
 
 	if (![m_window isKeyWindow])