I have a problem with GraphicsView

[cuijg]

When I drag the vertical line in A with the mouse, the movement of the vertical line in B is always delayed;

After I did this, the delay disappeared:

A.onMouseMove: 
{
     ... cur_scene_rect ...

     scene->setSceneRect(cur_scene_rect.left - 1,  ....);
     scene->setSceneRect(cur_scene_rect.left + 1,  ....);
     
     vline->setLine(....)
}

B.onMouseMoveFromA: 
{
     ... cur_scene_rect ...

     scene->setSceneRect(cur_scene_rect.left - 1,  ....);
     scene->setSceneRect(cur_scene_rect.left + 1,  ....);
     
     vline->setLine(....)
}

Why is this? Why does calling setSceneRect to change scene_rect make the delay disappear, but if we don't call setSceneRect twice and keep scene_rect unchanged, there will be a delay?

[cuijg]

I asked Gemini and they removed the method of calling setSceneRect twice and replaced it with the method of graphicsview->viewport()->update(), which has the same effect.