fixed holding alt while zooming zooms in by 2 increments.

scene/gui/view_panner.cpp

@@ -38,7 +38,9 @@ bool ViewPanner::gui_input(const Ref<InputEvent> &p_event, Rect2 p_canvas_rect)
 	Ref<InputEventMouseButton> mb = p_event;
 	if (mb.is_valid()) {
 		Vector2 scroll_vec = Vector2((mb->get_button_index() == MouseButton::WHEEL_RIGHT) - (mb->get_button_index() == MouseButton::WHEEL_LEFT), (mb->get_button_index() == MouseButton::WHEEL_DOWN) - (mb->get_button_index() == MouseButton::WHEEL_UP));
-		if (scroll_vec != Vector2()) {
+		// Moving the scroll wheel sends two events: one with pressed as true,
+		// and one with pressed as false. Make sure we only process one of them.
+		if (scroll_vec != Vector2() && mb->is_pressed()) {
 			if (control_scheme == SCROLL_PANS) {
 				if (mb->is_ctrl_pressed()) {
 					scroll_vec.y *= mb->get_factor();