CPU usage does not reduce when unfocusing editor if a tab is undocked

On Windows, if a tab is split out into its own window (undocked), CPU usage for the editor does not go into idle mode when the editor is unfocused.

This causes CPU usage to remain higher when the editor is unfocused if and only if multiple windows are opened.

Repro:

1. Open the editor
2. Open the out-of-process profiler and select "Editor" as the target"
3. In the editor (not-profiler), make sure everything is contained in the main window. No extra undocked tabs.
4. Click on another app to unfocus the editor
5. *Notice that the framerate in the profiler has dropped to save CPU usage*
6. Now focus the editor again. Drag the "Project" tab out so it goes into its own floating window.
7. Click on another app to unfocus the editor again

Result:

In the standalone profiler, you will see that the framerate remains high even though the editor is no longer focused.

Expected:

The framerate should drop just as when there were no undocked tabs.