Leafet maps only partially render when in non-displayed tabs

I am using the Tabs extension, https://www.mediawiki.org/wiki/Extension:Tabs, with a Leaflet map on one of the later tabs. When I load the page, the map starts to render, but then the Tabs logic comes into play, and the first tab is shown, and the map hidden as the page reshuffes. When I choose the Map tab, it just shows a partial map, with only a few tiles done, presumably because it was interrupted. See https://fanbase.johnbray.org.uk/Category:Novacon. It works if the map is in the first tab, but that does not fit in my design. How can I persuade it to finish?

Reproduce the issue on the linked Category:Novacon page with the MediaWiki Tabs extension and a Leaflet map in a later tab. Start by comparing rendering in the first tab with rendering after selecting the Map tab. Done means the later-tab map finishes rendering all tiles rather than remaining partial.