I wonder if the “hide preview” text could actually be hidden entirely, in favour of a tall thin button between the composer and the preview; something like this, perhaps?
AI-generated summary
The discussion revolves around moving the “hide preview” button in the composer to a line between the composer and preview area. barryvan suggests replacing the text with a tall, thin button, which could be grey or black to avoid adding more bright blue. codinghorror is opposed to the idea, citing the importance of the text in explaining the preview area and acting as a toggle. He also believes it would add unnecessary complexity to the composer. jomaxro supports barryvan’s suggestion, but codinghorror remains opposed, citing the need for clarity in the UI. barryvan responds with a revised design concept, but codinghorror ultimately concludes that the change would be a worse UI and is not worth the marginal benefit. notriddle suggests an alternative approach of separating the preview from the text box with a line without adding a button.
Adding more bright blue is definitely not a good idea IMO. And adding a dynamic preview resize bar is out of scope for what we’re doing here, that would be a fairly major feature to add, that we’ve never had.
I think if preview is going to float with a transparent / clear background, having the text there explaining what it is, and acting as a toggle, is quite important.
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I like @barryvan’s suggestion. I don’t believe he was suggesting a resizable preview area, just replacing the text “hide preview” with a button in the middle. Could be grey/black if blue is too much.
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Yes, but that’s still a sizable change, we’re mostly looking at cosmetics at this time, and as I already said
In general it is a good idea to remove text we have to translate, whenever we can, but I don’t think so in this particular specific case, at least not right now.
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The bright blue was just a placeholder – I’d likely make this thinner, grey, and with a nicer icon. 
And it shouldn’t be resizable – just a collapser. Something along these lines is probably a bit more subtle:
Not overly so, I think – it’s mainly changing the location of the “hide preview” link in the DOM, and some minor CSS styling.
From a user’s perspective, though, it is quite a large change, because rather than things just looking a little prettier, things will move – and so there could be some initial confusion.
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Separating the preview from the text box with a line doesn’t have to imply putting a button on that line.
In my opinion this adds a fair bit of new complexity to the composer, and the marginal benefit of slightly less translated text – since we already have that text area reserved at the bottom – is not worth it.
It’s arguably a worse UI, since the words are fairly clear and help identify the floating area on the right. Take that away and you make the UI less clear. What is that area on the right?
In summary, I would be quite opposed to this change.