Servo: Stuck inside a box while scrolling

[gterzian]

Steps to reproduce:

On mac

1. ./mach run https://en.wikipedia.org/wiki/Main_Page --release
2. Scroll to "today's featured artice".
3. Once with the cursor inside that box, try to scroll up, or down.
4. In both cases, you'll hit a "scroll bounce" as if hitting the limit of a page.
5. Go with the cursor to the left pannel, and you'll be able to scroll up again.

[CYBAI]

I just recorded a gif to show the behavior.

[CYBAI]

Here's the minimized test case.

<style>
.outer {
    border: 1px solid red;
    background-color: #efefef;
    overflow: auto;
}
</style>

<div class="outer">
  <h2>Today's featured picture</h2> 
</div>

After commenting the border: 1px solid red;, I can't scroll inside that div anymore.

Also, if there's only border and no overflow, I can't scroll either.

I think the issue is related to overflow with border.

cc @emilio

[KiChjang]

My educated guess is that the border is somehow calculated as part of the content, which may erroneously cause the weird scrolling behavior we see there.