Don't know the exact cause of the issue but you can fix it in multiple ways.
My previous explanation (maybe wrong) :
This isn't a bug, you put the background color
on a <span>
tag (in
position:relative
) inside a contenteditable div element and so the
span is on top of the contenteditable div.
I still think is related to the z-index
since we can see on the image below the red background on top of the Chrome focus border:

Remove position:relative
Removing position:relative
of the <span>
fix the issue:
.no-bug {
background-color: red;
}
<div contenteditable>
This has caret
<span class="no-bug">This has caret !</span>
this has caret
</div>
Adding a z-index
to the <span>
element
Adding a negative z-index
also fix the issue:
.no-bug {
background-color: red;
position: relative;
z-index: -10;
}
<div contenteditable>
This has caret
<span class="no-bug">This has caret !</span>
this has caret
</div>
Adding a display: inline-block
to the <span>
element
Adding a display: inline-block
(or display: block
) fix the issue:
.no-bug {
background-color: red;
position: relative;
display: inline-block;
}
<div contenteditable>
This has caret
<span class="no-bug">This has caret !</span>
this has caret
</div>
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