2 added 779 characters in body
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I would remove the comma completely. It's not required. Neither clause can exist on their own with your placements of the comma.

Replacing the commas with full stops makes this a little clearer.

  1. The worst case scenario. Is that X happens.
  2. The worse case scenario is. That X happens.
  1. The first sentence is missing a verb and an object.
  2. The second sentence is missing a subject. The first sentence, though grammatical, is semantically wrong.

You should be able to replace the commas with full stops (and potentially restore an ellipsis)

The worst case scenario. Is that X happens.The page displayed a list of vegetables, ordered by color
The worse case scenario isMy original example was bad, the comma was optional. That X happens

The comma here can be replaced with a full stop to form two sentences. Context is still required for the second sentence, but part of the sentence can be removed to avoid repetition.

My original example was bad. The comma [in that example] was optional.

I would remove the comma completely. It's not required. Neither clause can exist on their own.

Replacing the commas with full stops makes this a little clearer.

The worst case scenario. Is that X happens.
The worse case scenario is. That X happens.

I would remove the comma completely. It's not required. Neither clause can exist on their own with your placements of the comma.

Replacing the commas with full stops makes this a little clearer.

  1. The worst case scenario. Is that X happens.
  2. The worse case scenario is. That X happens.
  1. The first sentence is missing a verb and an object.
  2. The second sentence is missing a subject. The first sentence, though grammatical, is semantically wrong.

You should be able to replace the commas with full stops (and potentially restore an ellipsis)

The page displayed a list of vegetables, ordered by color
My original example was bad, the comma was optional.

The comma here can be replaced with a full stop to form two sentences. Context is still required for the second sentence, but part of the sentence can be removed to avoid repetition.

My original example was bad. The comma [in that example] was optional.

1
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I would remove the comma completely. It's not required. Neither clause can exist on their own.

Replacing the commas with full stops makes this a little clearer.

The worst case scenario. Is that X happens.
The worse case scenario is. That X happens.