After this feature continues to work, you will see an icon to expand the pop-up style editor window. Click it to bring up a full-screen editing window.
You can even set Gmail, by default, to always open a full-screen editing window. Click the More Options arrow in the lower right corner of the editing window.
From here, check the option ' Default to full-screen '.
You will still be able to open an editor window in a new tab or tab by holding down the 'Shift' key and clicking on the "Compose" button, or the expanded icon.