[REG 5.10 -> 5.11] Only half of the widget height is used

When a new page is loaded (e.g. in demobrowser) but the widget hasn't been redrawn yet, the widget seems to resize to half of its height - this can be seen by navigating to http://httpbin.org/delay/10 in demobrowser:

The new page then looks fine again, but in some cases (opening in a new tab, it seems), the new website only is rendered in the bottom half - for example, clicking a link on https://twitter.com/qtproject