IndefiniteSizeStrategy::findUsedFlexFraction needs to iterate all grid items that cross a flexible track. To do so it, takes the indices of the flex tracks, and for each one it uses GridIterator to iterate the items in that track.

Then, to avoid processing the same item multiple times, it checks that the item started in the current flex track, not in a previous one.

However, this is insufficient: it's not taking into account that an item can be in a single flex track, but span multiple tracks in the other axis.

This can make it very slow. Testcase: https://jsbin.com/hefajukaxo/1/edit?html,css,js,output

Chromium has been fixed in https://crbug.com/1026275