At work, I’m in between a few big projects, spending my time polishing the UI so it shines. One of those polish items has been to revamp how we do notification badges, and through that, I think I discovered a WebKit bug EDIT: Chrome bug. Upon further testing, this bug does not appear to be in Safari.

The simplest way to do a notification badge is like this:

1

2

Nice ‘n easy! Just slap a position: absolute inside a position: relative and everything’s pretty much good-to-go. The only downside to this is that is doesn’t quite look professional enough. A neat little detail to have is a little transparent space between the profile picture and the thing going on top of it, like in Discord.

Now, you can just fake this effect by slapping on a border: 5px solid var(--background-color) onto the the thing, but if the background color ever changes you have to animate this border property with the same curves, so for separation of concerns it’s just easier¹ to have the image be truly transparent in that area.

So! Let’s try to do exactly that. A quick dive into what exists out there in general brings up the mask property, with a specially crafted radial-gradient as the mask source. Let’s see how this looks, first without the indicator:

<style>
  /* irrelevant properties omitted for brevity */
  .take1 .pfp {
    border-radius: 50%;
    mask-image: radial-gradient(
      ellipse 20% 20% at 85% 85%,
      transparent 99%,
      white 100%
    );
  }
</style>

<div class="avatar take1">
  <img class="pfp" src="/uploads/randomlogo.png" />
</div>

Interesting!! What this is doing, effectively, is drawing a completely transparent circle at a specified location within the bounding box, using a transparent 99% and white 100% stop to effectively get a hard line instead of a gradient.

Unfortunately, this approach is not without problems.

Problem 1: It Looks Chunky

This becomes more apparent when we try to add the dot:

<style>
  .indicator {
    aspect-ratio: 1;
    border-radius: 50%;
  }

  .take1 .indicator {
    width: 30px;
    position: absolute;
    bottom: -3.5px;
    right: -3.5px;
  }
</style>
<div class="avatar take1">
  <img class="pfp" src="/uploads/randomlogo.png" />
  <div class="indicator green"></div>
</div>

Now, we can more clearly see that the masked-out part is not anti-aliased, unlike the circular borders of the profile picture and indicator. This is part of the bug I’m referring to in the post title.

Problem 2: It Clips

Things get worse if we try to add a second indicator (and not just from a design perspective, lol).

<style>
  .take2 .pfp {
    mask-image: radial-gradient(
      ellipse 20% 20% at 85% 85%,
      transparent 99%,
      white 100%
    );
  }

  .take2 img {
    border-radius: 50%;
  }

  .take2 .indicator {
    width: 30px;
    position: absolute;
  }

  .take2 .indicator1 {
    bottom: -3.5px;
    right: -3.5px;
  }

  .take2 .indicator2 {
    top: -3.5px;
    left: -3.5px;
  }
</style>

<div class="avatar take2">
  <div class="pfp">
    <img src="/uploads/randomlogo.png" />
    <div class="indicator indicator2 orange"></div>
  </div>
  <div class="indicator indicator1 green"></div>
</div>

Huh?? You may have many questions, such as “why not just put class="indicator2" outside the part that clips?” and “wait hold on why is is clipping like that in the first place?” The answers to which are:

1. React Component Hierarchy²: it’s simpler to make a component that says “I want to optionally add exactly one new indicator” on something than so say “I want to add a configurable number of indicators with variable positions”, because otherwise you have to somehow blend the masks which can get hairy³.
2. We’re Getting To That

It may have been wiser to bite the bullet and figure out how to reorganize the component hierarchy, but now I was curious to get to the bottom of this and figure out if I could do something about the first problem at the same time.

Poking around in developer tools, we can confirm that it’s just the mask property causing this clipping. With the mask on, affecting the bottom right corner, we suddenly get clipping in the top left. A panicked search for “mask clips things that it shouldn’t” brings us to the mask-clip property. Swell! Let’s just specify no-clip then, and:

Great!! Seems we’re done then, no? Ignoring the slight imperfection in the mask, of course. Well, if we happen to move .indicator2 to the right side, then…

<style>
  .take2 .indicator3 {
    top: -3.5px;
    right: -3.5px;
  }
</style>

<div class="avatar take2">
  <div class="pfp noclip">
    <img src="/uploads/randomlogo.png" />
    <div class="indicator indicator3 orange"></div>
  </div>
  <div class="indicator indicator1 green"></div>
</div>

HUH. WHAT. That’s strange, to say the least. All we did was change left: -3.5px to right: -3.5px and suddenly the top is shorn off?? Playing around a bit more and setting right: 10px, we see a clue:

That indentation in where it gets cut off is suspicious.. it’s almost like…

The Mask Is Repeating

That is, when the content goes outside the bounding box, a duplicate copy of the mask is placed there. Skimming the sidebar of the MDN tab where we found mask-clip, we do indeed see there is a mask-repeat property, whose default value implies this exact behavior. Attempting to set no-repeat on the mask, however, just results in it clipping again.

Problem 3: It’s Problem 1 Again, But Different

The designer comes back and says “that indicator in the top right is actually meant to be an unread message count, and should have a pill-shaped mask.” OK, no problem, some reading of MDN has us come across the mask-composite property, and a bit of fooling around with it later we have a prototype:

<style>
.take3 .pfp {
  border-radius: 50%;
  mask-image: radial-gradient(ellipse 20% 20% at 80% 20%, transparent 99%, white 100%), linear-gradient(to bottom, white 40%, transparent 41%), linear-gradient(to left, transparent 19%, white 20%);
  mask-composite: subtract;
}
</style>
<div class="avatar take3">
  <img class="pfp" src="/uploads/randomlogo.png"></div>
</div>

I could not honestly tell you how this works, only that it does. But as you can see from this image (and maybe even on your browser!), there’s a huge problem: not only is the circular part of the mask not anti-aliased, but it’s not even masking the anti-aliased pixels from the border-radius. This is the clearest demonstration of the WebKit bug I was promising to expose in this post, so here you go :)

A Better(?) Approach

Fortunately, that same day I found this bug I also came up with an approach that solves all of these problems, at a small cost to generated code size. But this post is long enough as it is already, and you’re probably fatigued reading this, so I think I’ll leave that for a follow-up post.

As an aside, Markdown is really nice for things like this. I can just include HTML right in with the rest of the formatted text and it just works?? Magical.