This is what I got in the admin content area after saving the site configuration (name, tagline) - obviously the TinyMCE toolbar items, etc. Look at the width 100% that is exclusive of the treeview width which is why the editor is going out of boundaries:
$('Form_EditForm').resetElements(); statusMessage('Saved site configuration', 'good'); if((typeof tinyMCE != 'undefined')) { tinymce.PluginManager.load("advcode", "../../../sapphire/thirdparty/tinymce-advcode/editor_plugin_src.js"); tinymce.PluginManager.load("ssbuttons", "../../../cms/javascript/tinymce_ssbuttons/editor_plugin_src.js"); tinyMCE.init({"friendly_name":"Default CMS","priority":"50","mode":"none","editor_selector":"htmleditor","width":"100%","auto_resize":false,"theme":"advanced","theme_advanced_layout_manager":"SimpleLayout","theme_advanced_toolbar_location":"top","theme_advanced_toolbar_align":"left","theme_advanced_toolbar_parent":"right","blockquote_clear_tag":"p","table_inline_editing":true,"safari_warning":false,"relative_urls":true,"verify_html":true,"spellchecker_rpc_url":"sapphire\/thirdparty\/tinymce-spellchecker\/rpc.php","language":"en","body_class":"typography","document_base_url":"http:\/\/localhost\/cms\/silverstripe\/","urlconverter_callback":"nullConverter","setupcontent_callback":"sapphiremce_setupcontent","cleanup_callback":"sapphiremce_cleanup","template_templates":[{"title":"Three column","src":"assets\/snippet.html","description":"A simple 3 column layout"}],"use_native_selects":true,"valid_elements":"@[id|class|style|title],+a[id|rel|rev|dir|tabindex|accesskey|type|name|href|target|title|class],-strong\/-b[class],-em\/-i[class],-strike[class],-u[class],#p[id|dir|class|align|style],-ol[class],-ul[class],-li[class],br,img[id|dir|longdesc|usemap|class|src|border|alt=|title|width|height|align],-sub[class],-sup[class],-blockquote[dir|class],-table[border=0|cellspacing|cellpadding|width|height|class|align|summary|dir|id|style],-tr[id|dir|class|rowspan|width|height|align|valign|bgcolor|background|bordercolor|style],tbody[id|class|style],thead[id|class|style],tfoot[id|class|style],#td[id|dir|class|colspan|rowspan|width|height|align|valign|scope|style],-th[id|dir|class|colspan|rowspan|width|height|align|valign|scope|style],caption[id|dir|class],-div[id|dir|class|align|style],-span[class|align|style],-pre[class|align],address[class|align],-h1[id|dir|class|align|style],-h2[id|dir|class|align|style],-h3[id|dir|class|align|style],-h4[id|dir|class|align|style],-h5[id|dir|class|align|style],-h6[id|dir|class|align|style],hr[class],dd[id|class|title|dir],dl[id|class|title|dir],dt[id|class|title|dir],@[id,style,class]","extended_valid_elements":"img[class|src|alt|title|hspace|vspace|width|height|align|onmouseover|onmouseout|name|usemap],iframe[src|name|width|height|align|frameborder|marginwidth|marginheight|scrolling],object[width|height|data|type],param[name|value],map[class|name|id],area[shape|coords|href|target|alt]","content_css":"cms\/css\/editor.css, themes\/blackcandy\/css\/editor.css","spellchecker_languages":"+English (United States)=en_US","plugins":"contextmenu,table,emotions,paste,-advcode,spellchecker,media,fullscreen,-ssbuttons","theme_advanced_buttons1":"bold,italic,underline,strikethrough,separator,justifyleft,justifycenter,justifyright,justifyfull,styleselect,formatselect,separator,bullist,numlist,outdent,indent,blockquote,hr,charmap","theme_advanced_buttons2":"undo,redo,separator,cut,copy,paste,pastetext,pasteword,spellchecker,separator,ssimage,ssflash,sslink,unlink,anchor,separator,advcode,fullscreen,separator,search,replace,selectall,visualaid,separator","theme_advanced_buttons3":"tablecontrols"}); }