Flashing signs when adding blank lines

[andmis]

Is your feature request related to a problem? Please describe. When editing a file, if you add a new line (by pressing enter or some other way), there is a brief flash before the sign is added. See this GIF:

Describe the solution you'd like It would be great if the sign were guessed/assumed/inferred synchronously (or at least, synchronously-enough that there is no flash).

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Edit: this is with set signcolumn=number.