Invalid CSS after editing isn't highlighted

[wbamberg]

Since the new Bob, if I edit a CSS value such that it is invalid, the "X" isn't shown unless I switch to a different value then switch back.

For example:

• visit https://developer.mozilla.org/en-US/docs/Web/CSS/filter
• select an option, select the text, and replace it with "bananas"

At this point I would expect to get the "X" and associated highlighting, telling me that "bananas" is invalid. But I don't.

• switch to a different option
• switch back to "bananas" -> now I see the "X"