[FIX] web: list: recompute default values for columns when visible

Before this commit, all the computation of the default widths for column
headers was done before the list was attached to the DOM, meaning that no
column was actually visible. This was a problem since columns that are
meant to stay unseen were taken into account in the calculation of relative
column widths.

Now, the same algorithm runs once the columns are visible so that invisible
columns can be properly excluded.

fixes #38744

Task 2076721