Echoing #395, there seems to be issues when the container resizes due to new elements / the window itself resizing. Not sure if this webpackbin is 100% replicating what I'm seeing on my end, but it does show how the map can fail to resize (both with and without a manual call to the mapbox .resize() function (try commenting out line 37).
I've also tried to not use flexbox (and instead have the container use inline-block) but the issue still arises.
https://www.webpackbin.com/bins/-KyrdSeFI0dpMmkHEuCx
Happy to add more details if necessary! Attached also is a screenshot of the issue on my current build; it arises when you toggle the tabs on the top left to hide/show the map view. If you drag to resize the window it corrects.

@romeboards Sorry for the late reply, you need to resize after you actually update the style of the container, see: https://www.webpackbin.com/bins/-KzVbA5I7NKGyiSNEVhL
^that wasn't much help as the link is broken.
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^that wasn't much help as the link is broken.