Sub editors do not gain focus when opened

Sub editors do not gain focus when opened. Ctrl+F4 (Close Major Tab) does not work until the open tab has gained focus, so it must be clicked before the hot-key works.

Reproduced in 4.8.3 binary and Main (//depot/UE4/Promotable-CL-2635815). The Close Major Tab keyboard shortcut was not available in previous versions.

1. Open any project
2. Create a new Blueprint of any type
3. Double-click the new Blueprint in the Content Browser to open it
4. Do NOT click inside the new sub-editor window
5. Press Ctrl+F4

Result:
The sub-editor is not closed. If you click inside the editor and try again, it will close as expected.

Expected:
The sub-editor gains focus automatically upon opening, and Ctrl+F4 works without needing to click inside the editor.