Some in-line comments, maybe more to come.

Here is a crash, after using the "save as" button (maybe not related, I don't have a backtrace yet).

xapantu@ntu /t/s/build> src/scratch-text-editor 
[INFO 02:22:03.881206] Application.vala:155: Scratch version: 2.2.1
[INFO 02:22:03.881260] Application.vala:157: Kernel version: 4.1.4-1-ARCH
[DEBUG 02:22:03.882757] Settings.vala:163: Loading settings from schema 'org.pantheon.scratch.saved-state'
[DEBUG 02:22:03.883261] Settings.vala:163: Loading settings from schema 'org.pantheon.scratch.settings'
[DEBUG 02:22:03.884352] Settings.vala:163: Loading settings from schema 'org.pantheon.scratch.services'
[DEBUG 02:22:03.959510] [Gtk] Connecting to session manager
[WARNING 02:22:03.965308] [Gtk] Attempting to add a widget with type GtkScrolledWindow to a GtkDialog, but as a GtkBin subclass a GtkDialog can only contain one widget at a time; it already contains a widget of type GtkBox
[WARNING 02:22:03.965550] [libpeas] Failed to enable unknown plugin loader 'gjs'
[DEBUG 02:22:03.991315] Settings.vala:163: Loading settings from schema 'org.pantheon.scratch.plugins.terminal'
[WARNING 02:22:04.011552] [libpeas] Failed to enable unknown plugin loader 'gjs'
[WARNING 02:22:04.045298] [GLib-GObject] The property GtkSettings:gtk-menu-images is deprecated and shouldn't be used anymore. It will be removed in a future version.
[FATAL 02:22:04.051353] [Gtk] gtk_header_bar_pack: assertion 'gtk_widget_get_parent (widget) == NULL' failed
[FATAL 02:22:04.157390] [Gtk] gtk_action_set_visible: assertion 'GTK_IS_ACTION (action)' failed
[DEBUG 02:22:04.157634] MainWindow.vala:673: open temporary file: Fichier texte à partir de 2015-09-24 01:28:31
[DEBUG 02:22:04.162284] sql-schema.vala:204: schema_version is 10
[INFO 02:22:04.206977] Document.vala:171: Opening "Nouveau document"
[DEBUG 02:22:04.312185] SplitView.vala:124: View added successfully
[WARNING 02:22:04.501706] [GLib-GObject] The property GtkCellRendererPixbuf:follow-state is deprecated and shouldn't be used anymore. It will be removed in a future version.
[DEBUG 02:22:04.502441] Settings.vala:163: Loading settings from schema 'org.pantheon.scratch.plugins.folder-manager'
[DEBUG 02:22:04.596172] sql-schema.vala:204: schema_version is 10
[INFO 02:22:04.606347] Document.vala:171: Opening "Nouveau document"
[WARNING 02:22:04.616528] DocumentView.vala:142: This Document was already opened! Not opening a duplicate!
[DEBUG 02:22:04.618693] sql-schema.vala:204: schema_version is 10
[INFO 02:22:04.625701] Document.vala:171: Opening "40_39.diff"
[FATAL 02:22:04.712490] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.718433] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.722147] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.727046] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.730144] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.732916] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.737164] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.741088] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.746381] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[FATAL 02:22:04.750260] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
*** BUG ***
In pixman_region32_init_rect: Invalid rectangle passed
Set a breakpoint on '_pixman_log_error' to debug

*** BUG ***
In pixman_region32_init_rect: Invalid rectangle passed
Set a breakpoint on '_pixman_log_error' to debug

*** BUG ***
In pixman_region32_init_rect: Invalid rectangle passed
Set a breakpoint on '_pixman_log_error' to debug

[FATAL 02:22:12.190305] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[INFO 02:22:13.454742] [Gtk] GtkDialog mapped without a transient parent. This is discouraged.
[WARNING 02:22:28.920017] Document.vala:742: query_info failed, but filename appears to be correct, allowing as new file
[WARNING 02:22:28.930654] Document.vala:587: Cannot create backup copy for file "test_backup": Erreur lors de l'obtention des informations du fichier « /tmp/test_backup » : Aucun fichier ou dossier de ce type
[FATAL 02:22:28.932460] [Gtk] gtk_text_view_get_editable: assertion 'GTK_IS_TEXT_VIEW (text_view)' failed
[INFO 02:22:29.064790] Document.vala:323: File "test_backup" saved succesfully
[DEBUG 02:22:29.064928] Document.vala:593: Backup file deleting: /tmp/test_backup~
[DEBUG 02:22:29.065068] Document.vala:595: Backup file doesn't exists: /tmp/test_backup~
fish: 'src/scratch-text-editor' terminated by signal SIGSEGV (Erreur de frontière d'adresse)