gnus-summary-update-hook

gnus-summary-update-hook is a variable defined in `gnus-sum.el'.
Its value is

nil

• This variable may be risky if used as a file-local variable.

Documentation:
*A hook called when a summary line is changed.
The hook will not be called if `gnus-visual' is nil.

The default function `gnus-summary-highlight-line' will
highlight the line according to the `gnus-summary-highlight'
variable.

You can customize this variable.