That also worked for me (as did un-maximising and re-maximising as opposed to minimising).
But i also noticed this - the min/max/close icons were missing (only the circle around the close button remained), see the pic below.
If you clicked where the controls should be they still worked, so I recon this might be a chromuim thing rather than an Anvil one, as I assume Anvil cannot alter how the Chromium app itself looks.
Also note that the buttons are still missing after min/maxing, so whilst that action fixed the missing scrollbar in Anvil, it did not fix chromium.
I am on a fully updated Ubuntu 20.4.