Improve button behavior when multiple mouse buttons are used at the same time

- To emit `pressed`, buttons require that the press was initiated while hovering.
- Controls can't grab focus from a mouse click if they're not hovered.
- Hovers are updated both before and after a handled mouse button event.

scene/main/viewport.cpp

@@ -1768,7 +1768,9 @@ void Viewport::_gui_input_event(Ref<InputEvent> p_event) {
 					Control *control = Object::cast_to<Control>(ci);
 					if (control) {
 						if (control->get_focus_mode() != Control::FOCUS_NONE) {
-							if (control != gui.key_focus) {
+							// Grabbing unhovered focus can cause issues when mouse is dragged
+							// with another button held down.
+							if (control != gui.key_focus && gui.mouse_over_hierarchy.has(control)) {
 								control->grab_focus();
 							}
 							break;