Changing scale ratio at runtime issues with Widgets

Steps to reproduce:

• run the application
• change scale ratio from settings and click Apply
• focus the Qt window again with mouse

First the window is too large (assuming scale was increased) and all text is blurry. However, when the window becomes active, it changes to normal size and this gets printed out:

The cached device pixel ratio value was stale on window expose.  Please file a QTBUG which explains how to reproduce.