Rounded button border animation

Question

I have a button for which I'm trying to animate the borders on hover. I have a working code, but the issue is that when you hover it, the radius on top left and bottom right starts at 0 and gets adjusted only when the animation finishes (you might need to zoom in to see what I'm talking about). It's not a huge deal, but makes the button look unpolished.

Is there a way to make sure those borders are rounded at the beginning?

Here's the code for the button:

.container {
  width: 100%;
  height: 200px;
  background: wheat;
  display: flex;
  justify-content: center;
  align-items: center;
}

.btn {
    position: relative;
    display: block;
    width: 180px;
    z-index: 3;
    cursor: pointer;
    border-radius: 4px;
    text-decoration: none;
    border: 1px solid black;
    text-align: center;
    padding: 8px 0;
    color: black;
}

.btn:before, .btn:after {
    display: block;
    content: ' ';
    border-top: none;
    border-right: none;
    border-bottom: none;
    border-left: none;
    position: absolute;
    width: 0;
    height: 0;
    opacity: 0;
    transition: opacity 200ms ease-in-out;
    border-radius: 4px;
}

.btn:before {
    top: -1px;
    left: -1px;
}

.btn:after {
    bottom: -1px;
    right: -1px;
}

.btn:hover:before {
    width: 180px;
    height: calc(100%);
    opacity: 1;
    border-top: 2px solid white;
    border-right: 2px solid white;
    transition: width 300ms cubic-bezier(.07, .62, .61, 1), height 150ms 300ms cubic-bezier(.07, .62, .61, 1);
}

.btn:hover:after {
    width: 180px;
    height: calc(100%);
    opacity: 1;
    border-bottom: 2px solid white;
    border-left: 2px solid white;
    transition: width 300ms cubic-bezier(.07, .62, .61, 1), height 150ms 300ms cubic-bezier(.07, .62, .61, 1);
}

<div class="container"><a href="" class='btn'>Hover me</a></div>

Answer 1

Found the solution, in the end it was pretty simple, just needed to add overflow:hidden to the .btn class.