[v4] <hr> not visible in Cards

[bradleygauthier]

When adding an <hr> inside a .card, the line does not appear. I believe this may be a bug.

The only way I found to make it appear, is by adding width: 100%; to the <hr>. But this doesn't seem like an intended feature.

[mdo]

Note for future me or someone who wants to fix this—probably the flex styles interfering.

[prateekgoel]

I did some research on this and found this:
https://stackoverflow.com/questions/34365271/hr-inside-container-with-display-flex-become-corrupted/34372979#34372979

Adding some margin left and right seems to fix this issue.

We can either add margin-left:0; and margin-right:0; or margin-left:1rem; and margin-right:1rem;

@mdo I would like to fix this.