Animating svg with "stroke-dashoffset" doesn't go smooth - css

Question

I am following this codepen to create a dialog animation. But the animation doesn't seem to go smooth like the reference when I set the bigger dialog width and height. It seems like the issue is with svg stroke-dashoffset values but I am not sure what values do I need to set. Here is the codepen that I reproduced.
<div id="modal-close-default" class="" uk-modal>
<div class="uk-modal-dialog custom-modal six uk-modal-body lab-border-7 uk-margin-auto-vertical">
..
<svg class="modal-svg" xmlns="http://www.w3.org/2000/svg" width="100%" height="100%" preserveAspectRatio="none">
<rect x="0" y="0" fill="none" width="600" height="376" rx="3" ry="3"></rect>
</svg>
</div>
</div>
#modal-close-default {
.uk-modal-dialog.custom-modal {
..
}
.modal-svg {
position: absolute;
top: 0;
left: 0;
height: 100%;
width: 100%;
border-radius: 3px;
rect {
stroke: #fff;
stroke-width: 2px;
stroke-dasharray: 976; // total of dialog width and height (not sure what value to add)
stroke-dashoffset: 976;
}
}
&.uk-open>.uk-modal-dialog.custom-modal {
..
.modal-svg {
rect {
animation: sketchIn .5s .3s cubic-bezier(0.165, 0.840, 0.440, 1.000) forwards; // animation is not smooth
}
}
}
}
#keyframes sketchIn {
0% {
stroke-dashoffset: 976;
}
100% {
stroke-dashoffset: 0;
}
}

Answer 1

You miscalculated the length of the perimeter of the rectangle
If it's rough, then you need to consider this (width +height) * 2 = 1952px
The JS method getTotalLength() will help you calculate exactly the perimeter, taking into account the roundings.
Happened with rounding ~= 1946px
.modal-svg {
position: absolute;
top: 0;
left: 0;
border-radius: 3px;
}
rect {
stroke: silver;
stroke-width: 6px;
stroke-dasharray: 1946; // total of dialog width and height (not sure what value to add)
stroke-dashoffset: 1946;
animation: sketchIn 5s .3s cubic-bezier(0.165, 0.840, 0.440, 1.000) forwards; // animation is not smooth
}
#keyframes sketchIn {
0% {
stroke-dashoffset: 1946;
}
100% {
stroke-dashoffset: 0;
}
}
<div id="modal-close-default" class="uk-modal">
<div class="uk-modal-dialog custom-modal six uk-modal-body lab-border-7 uk-margin-auto-vertical">
<svg class="modal-svg" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 600 376" width="600" height="376" >
<rect id="rect" x="0" y="0" fill="none" width="600" height="376" rx="3" ry="3"></rect>
</svg>
</div>
</div>
<script>
let total = rect.getTotalLength();
console.log(total)
</script>