In bug 725376, we limited the maximum level of nested balancing columns to 2 levels. With column-span enabled, the helper function [`ColumnBalancingDepth`](https://searchfox.org/mozilla-central/rev/428cf94f4ddfb80eebc6028023a7d89eb277791b/layout/generic/nsColumnSetFrame.cpp#279-289) doesn't compute the correct depth because we now have `ColumnSetWrapperFrame` wrapping `nsColumnSetFrame`. However, the saved test case https://bug725376.bmoattachments.org/attachment.cgi?id=595472 doesn't hang on Firefox Nightly (2019-08-19)
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In bug 725376, we limited the maximum level of nested balancing columns to 2 levels. With column-span enabled, the maximum balancing level doesn't seem to help. It takes long time to load this test case https://bug725376.bmoattachments.org/attachment.cgi?id=595472