White light curves of HAT-P-18 b. Top left: the JWST transit (ID: 2734o1-NIRISS, proposal ID 2734, observation one, and observed with the JWST/NIRISS instrument). Top right: the 2015 HST/WFC3 transit (program ID 14099). Bottom: the 2016 (left) and 2017 (right) HST/WFC3 transits (program ID 14260). In each panel, the upper subpanel shows the relative flux light curve and the lower subpanel shows the corresponding residuals; unfilled blue points represent the data and the black line indicates the best-fitting model. Red points and the shaded areas mark the spot-crossing events: the clear event near the bottom of the JWST transit, and the low-level feature near the bottom of the 2015 HST transit.