Fix dead space in combobox dropdown

[charliepark]

This patches up the combobox by adding a pseudo element before the button, and then makes the button full-height.
Have checked it across browsers and all seems well.

It was looking a little faint to me, but here's a quick comparison with the current style:

current:

with pseudo element:

Closes #3185

[vercel]

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Project	Deployment	Actions	Updated (UTC)
console	Ready	Preview	Apr 17, 2026 5:58pm

[charliepark]

This is with stroke-default, if we feel like this is better:

[david-crespo]

Works great, thanks! I like the line better dim. It's true that when you pay attention to it, it looks a little weird, but when you're just glancing at the button casually, it gives the button a nice boundary without calling attention to itself.