Border-radius and overflow hidden with child background - css

Question

I've got a problem with border-radius on wrapper that contains an overflow hidden.
I use a before pseudo element (pink background) to fill the wrapper's background. The wrapper has already a background (blue).
#wrapper {
background: blue;
border: 2px solid pink;
border-radius: 12px;
height: 90px;
overflow: hidden;
position: relative;
width: 300px;
}
#wrapper::before {
background: pink;
content: '';
display: block;
height: 100%;
left: 0;
position: absolute;
top: 0;
width: 50%;
}
<div id="wrapper"></div>
With this example, we can see an unwanted blue pixel on the top and bottom left corner.
The pseudo element must be in position absolute to apply animation. I removed the animation for the example.
How can I fix this?

Answer 1

A fix is here. Apply overflow:hidden an width:300px to the outer div #container.
#container {
width: 300px;
overflow: hidden;
border-radius: 12px;
}
#wrapper {
height: 90px;
background: blue;
border-radius: 12px;
position: relative;
box-sizing: border-box;
border: 2px solid pink;
}
#wrapper::before {
background: pink;
content: '';
display: block;
height: 100%;
right: -30px;
position: absolute;
top: 0;
width: 30px;
border-radius: 50%;
transition: transform 0.3s;
}
#wrapper:hover::before {
transform: scale3D(10, 10, 1);
}
<div id="container">
<div id="wrapper"></div>
</div>

Answer 2

You found a really interesting rendering issue. My idea to solve it, is switch the colors and logic a little:
#wrapper {
background: pink;
border: 2px solid pink;
border-radius: 12px;
height: 90px;
overflow: hidden;
position: relative;
width: 300px;
}
#wrapper::before {
background: blue;
content: '';
display: block;
height: 100%;
right: 0;
position: absolute;
top: 0;
width: 50%;
}
<div id="wrapper"></div>