What
When a message you send is very long, the conversation view now shows only a limited number of its lines. The rest is replaced by a marker counting the hidden lines and characters, and selecting the message expands it to show everything. The check for whether a message is too long is redone when your terminal width changes.
Why
Large pasted prompts should no longer fill your screen and push the rest of the conversation out of view.
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The writer flagged doubt
How many lines are shown before shortening, and whether some modes turn this off, is not known.