Stretch factors and size basis should be initialized when sizer is created.

[sccolbert]

For BoxLayout and SplitLayout, these properties should be initialized when the sizer is created (if possible). In some cases, the widget will be resized before the fit request is processed. This means the initial layout will have stale data. In the case of the SplitLayout, this causes the initial layout to be completely wrong when one widget has a stretch factor of zero.

https://github.com/phosphorjs/phosphor/blob/master/packages/widgets/src/splitlayout.ts#L297