Looking at theme-editor.php code it calls get_themes() (fills global $wp_themes variable with themes, including full lists of theme files) and then simply picks first file out of files from current theme.
It jumps to CSS because PHP and CSS files are stored separately and when arrays merge CSS comes on top.
So you can just rename your file so it isn’t first in list.
More complex way would be to hook and run get_themes() earlier and then manually rearrange files in $wp_themes. I am not good with admin area, have trouble finding fitting hook to run it conditionally on editor page alone.