diff --git a/assets-cg/icon-sel.png b/assets-cg/icon-sel.png
index 2d6b250..738a9f9 100644
Binary files a/assets-cg/icon-sel.png and b/assets-cg/icon-sel.png differ
diff --git a/assets-cg/icon-uns.png b/assets-cg/icon-uns.png
index 59586b4..acc867a 100644
Binary files a/assets-cg/icon-uns.png and b/assets-cg/icon-uns.png differ
diff --git a/assets-cg/icon.xcf b/assets-cg/icon.xcf
index d57fd44..caa65b5 100644
Binary files a/assets-cg/icon.xcf and b/assets-cg/icon.xcf differ
diff --git a/src/heditor.c b/src/heditor.c
index dc9186f..0c67174 100644
--- a/src/heditor.c
+++ b/src/heditor.c
@@ -27,6 +27,7 @@ heditor *heditor_create(jwidget *parent)
     e->source = NULL;
     e->scroll = 0;
     e->cursor = 0;
+    e->insert = false;
     e->visible_lines = 0;
     e->bytes_per_line = 8;
     e->line_spacing = 3;
@@ -117,6 +118,14 @@ bool heditor_move_to(heditor *e, int target_cursor)
     return changed;
 }
 
+void heditor_set_insert_mode(heditor *e, bool insert)
+{
+    e->insert = false;
+    if(e->source && e->source->intf->cap & SOURCE_EXPAND)
+        e->insert = insert;
+    e->widget.update = true;
+}
+
 void heditor_set_source(heditor *e, source_t *source)
 {
     e->source = source;
@@ -208,9 +217,15 @@ static void heditor_poly_render(void *e0, int x, int y)
 
             int fg = C_BLACK;
             if(e->cursor == o) {
-                drect(bytes_x - 1, line_y - 1, bytes_x + text_w,
-                    line_y + e->font->line_height, C_BLACK);
-                fg = C_WHITE;
+                if(e->insert) {
+                    drect(bytes_x - 2, line_y - 1, bytes_x - 1,
+                        line_y + e->font->line_height, C_BLACK);
+                }
+                else {
+                    drect(bytes_x - 1, line_y - 1, bytes_x + text_w,
+                        line_y + e->font->line_height, C_BLACK);
+                    fg = C_WHITE;
+                }
             }
             dtext(bytes_x, line_y, fg, byte);
 
diff --git a/src/heditor.h b/src/heditor.h
index 7605d82..6feacca 100644
--- a/src/heditor.h
+++ b/src/heditor.h
@@ -23,6 +23,8 @@ typedef struct {
     int scroll;
     /* Current cursor position in source, in bytes */
     int cursor;
+    /* Current insertion mode (false=overwrite, true=insert) */
+    bool insert;
     /* Number of visible lines */
     int8_t visible_lines;
     /* Number of bytes printed per line */
@@ -47,6 +49,10 @@ void heditor_set_source(heditor *e, source_t *source);
 /* Scroll to the requested cursor position. */
 bool heditor_move_to(heditor *e, int offset);
 
+/* Change the insertion mode; true for insertion, false for overwrite. If the
+   source doesn't support insertion, this function is a no-op. */
+void heditor_set_insert_mode(heditor *e, bool insert);
+
 /* Trivial properties */
 void heditor_set_font(heditor *e, font_t const *font);
 void heditor_set_line_spacing(heditor *e, int line_spacig);
diff --git a/src/main.c b/src/main.c
index 66c4f54..128526d 100644
--- a/src/main.c
+++ b/src/main.c
@@ -15,38 +15,170 @@
 #include <stdlib.h>
 #include <string.h>
 
-static void update_status(jlabel *status, jlabel *title, source_t *s)
+// Global application data //
+
+struct {
+    /* Title widget, status bar widget, f-keys widget. */
+    jlabel *title;
+    jlabel *status;
+    jfkeys *fkeys;
+    /* Source info widget. */
+    jlabel *source_info;
+
+    /* Current data source. */
+    source_t *source;
+    /* Current edition mode. */
+    bool insert;
+
+    /* Current view. */
+    int view;
+    /* Current position in the menus. */
+    int fkmenu;
+} app;
+
+enum {
+    VIEW_EDITOR,
+    VIEW_INFO,
+    VIEW_STATS,
+};
+enum {
+    FK_EDITOR,
+    FK_EDITOR_SOURCE,
+    FK_EDITOR_OPEN,
+    FK_VIEW,
+};
+
+// General UI update functions //
+
+static void fkeys_sprintf(jfkeys *fk, char const *fmt, ...)
 {
+    static char str[128];
+    va_list args;
+    va_start(args, fmt);
+    vsnprintf(str, sizeof str, fmt, args);
+    va_end(args);
+    jfkeys_set(fk, str);
+}
+
+static void update_fkeys(void)
+{
+    if(app.fkmenu == FK_EDITOR) {
+        if(!app.source)
+            jfkeys_set(app.fkeys, "/OPEN;;;;;");
+        else {
+            bool can_insert = (app.source->intf->cap & SOURCE_EXPAND) != 0;
+            fkeys_sprintf(app.fkeys, "/OPEN;/SOURCE;%s%s;@GOTO;;#VIEW",
+                can_insert ? "@" : "#",
+                app.insert ? "INSERT" : "OVERW");
+        }
+    }
+    else if(app.fkmenu == FK_EDITOR_SOURCE) {
+        jfkeys_set(app.fkeys, "@SAVE;@SAVE AS;;;;");
+    }
+    else if(app.fkmenu == FK_EDITOR_OPEN) {
+        jfkeys_set(app.fkeys, "@FILE;@LAZY;@MEMORY;@NEW");
+    }
+    else if(app.fkmenu == FK_VIEW) {
+        fkeys_sprintf(app.fkeys, ";;;%sEDITOR;%sINFO;%sSTATS",
+            app.view == VIEW_EDITOR ? "#" : "@",
+            app.view == VIEW_INFO   ? "#" : "@",
+            app.view == VIEW_STATS  ? "#" : "@");
+    }
+}
+
+static void update_status(void)
+{
+    /* TODO: (*) and (**) markers on title/status bar */
+    source_t *s = app.source;
+
     if(!s) {
-        jlabel_set_text(status, "-");
-        jlabel_set_text(title, "Hex Editor");
+        jwidget_set_visible(app.status, false);
+        jlabel_set_text(app.title, "Hex Editor");
         return;
     }
 
     size_t size = source_size(s);
+    jwidget_set_visible(app.status, true);
 
     if(s->origin)
-        jlabel_asprintf(title, "%s (%d B) - Hex Editor", s->origin, size);
+        jlabel_asprintf(app.title, "%s (%d B) - Hex Editor", s->origin, size);
     else
-        jlabel_set_text(title, "(nil) - Hex Editor");
+        jlabel_set_text(app.title, "(nil) - Hex Editor");
 
-    jlabel_asprintf(status, "Buffer: @%X %d/%d",
+    jlabel_asprintf(app.status, "Buffer: @%X %d/%d",
         s->buf_offset, s->buf->data_size, s->buf->mem_size);
 }
 
+static void update_source_info(void)
+{
+    source_t *s = app.source;
+
+    if(!s) {
+        jlabel_set_text(app.source_info, "No source.");
+        return;
+    }
+
+    char caps[48];
+    memset(caps, 0, sizeof caps);
+
+    if(s->intf->cap & SOURCE_WRITE)
+        strcat(caps, " WRITE");
+    if(s->intf->cap & SOURCE_EXPAND)
+        strcat(caps, " EXPAND");
+    if(s->intf->cap & SOURCE_SHRINK)
+        strcat(caps, " SHRINK");
+
+    jlabel_asprintf(app.source_info,
+        "%s\n"
+        "Size: %d bytes\n"
+        "Front buffer: %p, size %d/%d (x%d)\n"
+        "Front buffer maps to: @%d, size TODO\n"
+        "Capabilities: %s",
+        s->origin ? s->origin : "<unnamed source>",
+        source_size(s),
+        s->buf->mem, s->buf->data_size, s->buf->mem_size, s->buf->block_size,
+        s->buf_offset, /* s->buf_size, */
+        caps + 1);
+}
+
+static jwidget *make_popup(void *parent0, int w, int h)
+{
+    J_CAST(parent)
+
+    jwidget *popup = jwidget_create(parent);
+    if(!popup)
+        return NULL;
+
+    jwidget_set_floating(popup, true);
+    jwidget_set_border(popup, J_BORDER_SOLID, 2, C_BLACK);
+    jwidget_set_padding(popup, 4, 4, 4, 4);
+    jwidget_set_background(popup, C_WHITE);
+    jwidget_set_visible(popup, false);
+    jwidget_set_fixed_size(popup, w, h);
+
+    popup->x = (DWIDTH - w) / 2;
+    popup->y = (DHEIGHT - h) / 2;
+    return popup;
+}
+
 void hex_view(void)
 {
-    source_t *s = NULL;
+    app.source = NULL;
+    app.insert = false;
+    app.view = VIEW_EDITOR;
+    app.fkmenu = FK_EDITOR;
 
     jscene *scene = jscene_create_fullscreen(NULL);
     jlabel *title = jlabel_create("Hex Editor", scene);
     jwidget *stack = jwidget_create(scene);
-    jfkeys *fkeys = jfkeys_create("@JUMP;@OPEN",scene);
+    jfkeys *fkeys = jfkeys_create(";;;;;",scene);
 
     if(!scene || !title || !stack || !fkeys) {
         jwidget_destroy(scene);
         return;
     }
+    app.title = title;
+    app.fkeys = fkeys;
 
     jwidget_set_background(title, C_BLACK);
     jlabel_set_text_color(title, C_WHITE);
@@ -62,58 +194,51 @@ void hex_view(void)
 
     jwidget *tab = jwidget_create(NULL);
     heditor *mem = heditor_create(tab); /* previously 321x153 */
+    jinput *goto_input = jinput_create("Goto: ", 12, tab);
     jlabel *status = jlabel_create("", tab);
+    app.status = status;
 
-    jwidget_set_margin(mem, 3, 0, 3, 0);
+    jwidget_set_margin(mem, 2, 0, 0, 0);
     jwidget_set_stretch(mem, 1, 1, false);
 
     jwidget_set_stretch(status, 1, 0, false);
+    jwidget_set_padding(status, 2, 0, 0, 0);
+    jwidget_set_borders(status, J_BORDER_SOLID, C_BLACK, 1, 0, 0, 0);
+
+    jwidget_set_stretch(goto_input, 1, 0, false);
+    jwidget_set_visible(goto_input, false);
 
     jlayout_set_vbox(tab)->spacing = 3;
     jwidget_add_child(stack, tab);
     jwidget_set_stretch(tab, 1, 1, false);
 
-    update_status(status, title, s);
-    jscene_show_and_focus(scene, mem);
-
     // File selection tab //
 
     jfileselect *fileselect = jfileselect_create(stack);
     jfileselect_set_show_file_size(fileselect, true);
     jwidget_set_stretch(fileselect, 1, 1, false);
 
-    // Goto popup //
+    // Source info popup //
 
-    jwidget *popup_goto = jwidget_create(scene);
-    jwidget_set_floating(popup_goto, true);
-    jwidget_set_border(popup_goto, J_BORDER_SOLID, 2, C_BLACK);
-    jwidget_set_padding(popup_goto, 4, 4, 4, 4);
-    jwidget_set_background(popup_goto, C_WHITE);
-    jwidget_set_visible(popup_goto, false);
-    jwidget_set_fixed_size(popup_goto, DWIDTH * 3 / 4, DHEIGHT / 2);
-    popup_goto->x = DWIDTH / 8;
-    popup_goto->y = DHEIGHT / 4;
-    jlayout_set_vbox(popup_goto)->spacing = 3;
+    jwidget *popup_source_info = make_popup(scene, DWIDTH*3/4, DHEIGHT/2);
+    jlayout_set_vbox(popup_source_info)->spacing = 3;
 
+    jlabel *source_info = jlabel_create("", popup_source_info);
+    jlabel_set_wrap_mode(source_info, J_WRAP_WORD);
+    jlabel_set_line_spacing(source_info, 2);
+    jlabel_set_block_alignment(source_info, J_ALIGN_LEFT, J_ALIGN_TOP);
+    jwidget_set_stretch(source_info, 1, 1, false);
 
-    jinput *goto_input = jinput_create("Go to: ", 12, popup_goto);
-    jwidget_set_stretch(goto_input, 1, 0, false);
+    // Initial state //
 
-    jlabel *goto_help = jlabel_create(
-        "In hexadecimal. Use [ALPHA] to type letters.\n"
-        "Prefix with [+] or [-] to move relative to the current position.",
-        popup_goto);
-    jlabel_set_wrap_mode(goto_help, J_WRAP_WORD);
-    jlabel_set_line_spacing(goto_help, 2);
-    jlabel_set_block_alignment(goto_help, J_ALIGN_LEFT, J_ALIGN_TOP);
-    jwidget_set_stretch(goto_help, 1, 1, false);
+    update_status();
+    update_fkeys();
+    jscene_show_and_focus(scene, mem);
 
     // Event handling //
 
     int key = 0;
-    while(key != KEY_EXIT)
-    {
-        bool input_focus = (jscene_focused_widget(scene) == goto_input);
+    while(1) {
         jevent e = jscene_run(scene);
 
         if(e.type == JSCENE_PAINT) {
@@ -124,50 +249,146 @@ void hex_view(void)
         if(e.type == JINPUT_VALIDATED) {
             /* Parse string into hexa */
             char const *str = jinput_value(goto_input);
-            uint32_t target = strtoul(str, NULL, 16);
-            heditor_move_to(mem, target);
+            int mode = (*str == '+') ? 1 : (*str == '-') ? -1 : 0;
+            int target = strtoul(str + (mode != 0), NULL, 16);
+            heditor_move_to(mem, mode ? mem->cursor + target * mode : target);
         }
         if(e.type == JINPUT_VALIDATED || e.type == JINPUT_CANCELED) {
-            jwidget_set_visible(popup_goto, false);
+            jwidget_set_visible(goto_input, false);
             jscene_show_and_focus(scene, mem);
         }
         if(e.type == JFILESELECT_LOADED) {
             char const *path = jfileselect_current_folder(fileselect);
             jlabel_asprintf(title, "Browsing: %s", path ? path : "(nil)");
         }
-        if(e.type == JFILESELECT_CANCELED || e.type == JFILESELECT_VALIDATED) {
+        if(e.type == JFILESELECT_CANCELED) {
             jscene_show_and_focus(scene, mem);
-
-            if(e.type == JFILESELECT_VALIDATED) {
-                char const *file = jfileselect_selected_file(fileselect);
-                if(file) {
-                    if(s)
-                        source_free(s);
-                    s = source_loaded_file_open(file);
-                    if(s) {
-                        source_load(s, 0, 2048);
-                    }
-                    heditor_set_source(mem, s);
-                }
+            update_status();
+        }
+        if(e.type == JFILESELECT_VALIDATED) {
+            char const *file = jfileselect_selected_file(fileselect);
+            if(file) {
+                if(app.source)
+                    source_free(app.source);
+                app.source = source_loaded_file_open(file);
+                app.fkmenu = FK_EDITOR;
+                app.insert = false;
+                source_load(app.source, 0, 2048);
+                heditor_set_source(mem, app.source);
+                heditor_set_insert_mode(mem, app.insert);
+                update_fkeys();
             }
-            update_status(status, title, s);
+            jscene_show_and_focus(scene, mem);
+            update_status();
         }
         if(e.type == HEDITOR_CHANGED) {
-            update_status(status, title, s);
+            update_status();
         }
 
+        // Fkey menu navigation //
+
         if(e.type != JSCENE_KEY || e.key.type == KEYEV_UP) continue;
         key = e.key.key;
 
-        if(key == KEY_F1 && stack->layout_stack.active == 0 && !input_focus) {
-            jinput_clear(goto_input);
-            jwidget_set_visible(popup_goto, true);
-            jscene_set_focused_widget(scene, goto_input);
+        if(jscene_focused_widget(scene) == goto_input)
+            continue;
+
+        if(app.fkmenu == FK_EDITOR) {
+            if(key == KEY_F1) {
+                app.fkmenu = FK_EDITOR_OPEN;
+                update_fkeys();
+            }
+            else if(key == KEY_F2 && app.source) {
+                app.fkmenu = FK_EDITOR_SOURCE;
+                update_fkeys();
+            }
+            else if(key == KEY_F3 && app.source) {
+                if(app.source->intf->cap & SOURCE_EXPAND) {
+                    app.insert = !app.insert;
+                    heditor_set_insert_mode(mem, app.insert);
+                    update_fkeys();
+                }
+            }
+            else if(key == KEY_F4 && app.source) {
+                jinput_clear(goto_input);
+                jscene_show_and_focus(scene, goto_input);
+            }
+            else if(key == KEY_F6 && app.source) {
+                app.fkmenu = FK_VIEW;
+                update_fkeys();
+            }
         }
-        if(key == KEY_F2 && !input_focus) {
-            jfileselect_browse(fileselect, "/");
-            jscene_show_and_focus(scene, fileselect);
+        else if(app.fkmenu == FK_EDITOR_SOURCE) {
+            if(key == KEY_F1 && app.source) {
+                /* TODO: Source save */
+            }
+            else if(key == KEY_F2 && app.source) {
+                /* TODO: Source save as */
+            }
+            else if(key == KEY_EXIT) {
+                app.fkmenu = FK_EDITOR;
+                update_fkeys();
+            }
         }
+        else if(app.fkmenu == FK_EDITOR_OPEN) {
+            if(key == KEY_F1 || key == KEY_F2) {
+                /* TODO: Check if current source is unsaved? */
+                jfileselect_browse(fileselect, "/");
+                jscene_show_and_focus(scene, fileselect);
+            }
+            else if(key == KEY_F3) {
+                /* TODO: Memory browser */
+            }
+            else if(key == KEY_F4) {
+                /* TODO: New empty source */
+            }
+            else if(key == KEY_EXIT) {
+                app.fkmenu = FK_EDITOR;
+                update_fkeys();
+            }
+        }
+        else if(app.fkmenu == FK_VIEW) {
+            if(key == KEY_F4) {
+                jscene_show_and_focus(scene, mem);
+                app.fkmenu = FK_EDITOR;
+                update_fkeys();
+            }
+            else if(key == KEY_F5) {
+                /* TODO: Show info panel */
+                // jscene_show_and_focus(scene, tab_info);
+                // app.fkmenu = FK_INFO;
+                // update_fkeys();
+            }
+            else if(key == KEY_F6) {
+                /* TODO: Show statistics panel */
+                // jscene_show_and_focus(scene, tab_stats);
+                // app.fkmenu = FK_STATS;
+                // update_fkeys();
+            }
+            else if(key == KEY_EXIT) {
+                // if(current_tab == tab_editor)
+                    app.fkmenu = FK_EDITOR;
+                // else if(current_tab == tab_info)
+                //     app.fkmenu = FK_INFO;
+                // else if(current_tab == tab_stats)
+                //     app.fkmenu = FK_STATS;
+                update_fkeys();
+            }
+        }
+
+#if 0
+        if(key == KEY_F3 && !input_focus) {
+            if(!jwidget_visible(popup_source_info)) {
+                update_source_info();
+                jwidget_set_visible(popup_source_info, true);
+                jscene_set_focused_widget(scene, source_info);
+            }
+            else {
+                jwidget_set_visible(popup_source_info, false);
+                jscene_show_and_focus(scene, mem);
+            }
+        }
+#endif
     }
     jwidget_destroy(scene);
 }
diff --git a/src/source.c b/src/source.c
index acf4dd6..de30c3b 100644
--- a/src/source.c
+++ b/src/source.c
@@ -30,8 +30,8 @@ size_t source_size(source_t *s)
 
 ssize_t source_load(source_t *s, off_t offset, size_t segment_size)
 {
-    if(!buffer_resize(s->buf, segment_size))
-        return false;
+    if(!s || !buffer_resize(s->buf, segment_size))
+        return -1;
 
     s->buf_offset = offset;
     ssize_t rc = s->intf->read(s->cookie, s->buf->mem, offset, segment_size);