fullscreen: info/EXIF, fullscreen, slideshow, Esc back (ot0)

M4 (task ot0): fullscreen + slideshow + info overlay.

- src/info.{c,h}: plain-C EXIF gather via libexif (camera, lens, focal,
  aperture, shutter, ISO, datetime, orientation) + file info (dims, format,
  size). info_format() produces the overlay text. Unit-tested.
- src/window.c: fullscreen toggle (f), slideshow (S, 3s auto-advance), info
  overlay (i, GtkLabel in GtkOverlay, auto-hide 5s), Esc contextual back
  (fullscreen→unfullscreen→large→grid→quit). Stack wrapped in GtkOverlay.
  ggaze_window_get_stack() accessor for tests.
- src/shortcuts.c: f, S, i, Escape bindings.
- meson.build: libexif dep added.
- tests: unit test_info (EXIF extraction + orientation tag). 14/14 green,
  ASan clean.

5 files changed, 374 insertions, 6 deletions

diff --git a/src/info.c b/src/info.c
new file mode 100644
index 0000000..664ea64
--- /dev/null
+++ b/src/info.c
@@ -0,0 +1,171 @@
+/*:*
+ * ggaze — image info / EXIF gather
+ *
+ * Gathers file info (via GFileInfo + GdkPixbuf for dims) and EXIF tags (via
+ * libexif) into a GgazeInfo struct. Plain-C, no GtkWidget.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "info.h"
+
+#include <gdk-pixbuf/gdk-pixbuf.h>
+#include <libexif/exif-data.h>
+#include <libexif/exif-tag.h>
+
+static char *
+_dup_exif_value(ExifData *p_data, ExifTag e_tag) {
+   if (p_data == NULL) {
+      return (NULL);
+   }
+   ExifEntry *p_entry = exif_data_get_entry(p_data, e_tag);
+   if (p_entry == NULL) {
+      return (NULL);
+   }
+   char c_buf[256];
+   exif_entry_get_value(p_entry, c_buf, sizeof(c_buf));
+   if (c_buf[0] == '\0') {
+      return (NULL);
+   }
+   return (g_strdup(c_buf));
+}
+
+static char *
+_dup_camera(ExifData *p_data) {
+   char *c_make  = _dup_exif_value(p_data, EXIF_TAG_MAKE);
+   char *c_model = _dup_exif_value(p_data, EXIF_TAG_MODEL);
+   char *c_out   = NULL;
+   if (c_make != NULL && c_model != NULL) {
+      c_out = g_strdup_printf("%s %s", c_make, c_model);
+   } else if (c_make != NULL) {
+      c_out = g_strdup(c_make);
+   } else if (c_model != NULL) {
+      c_out = g_strdup(c_model);
+   }
+   g_free(c_make);
+   g_free(c_model);
+   return (c_out);
+}
+
+static int
+_get_orientation(ExifData *p_data) {
+   if (p_data == NULL) {
+      return (0);
+   }
+   ExifEntry *p_entry = exif_data_get_entry(p_data, EXIF_TAG_ORIENTATION);
+   if (p_entry == NULL || p_entry->components == 0 || p_entry->data == NULL) {
+      return (0);
+   }
+   /* Orientation is a SHORT (2 bytes, big/little endian per the data's byte
+    * order). */
+   ExifByteOrder e_order = exif_data_get_byte_order(p_data);
+   return ((int)exif_get_short(p_entry->data, e_order));
+}
+
+GgazeInfo *
+info_new(GFile *p_file) {
+   g_return_val_if_fail(G_IS_FILE(p_file), NULL);
+   char *c_path = g_file_get_path(p_file);
+   if (c_path == NULL) {
+      return (NULL);
+   }
+
+   GgazeInfo *p_info = g_new0(GgazeInfo, 1);
+
+   /* File info: size + content type. */
+   GError    *p_err = NULL;
+   GFileInfo *p_fi =
+      g_file_query_info(p_file, "standard::size,standard::content-type",
+                        G_FILE_QUERY_INFO_NONE, NULL, &p_err);
+   if (p_fi != NULL) {
+      p_info->i_size   = (gint64)g_file_info_get_size(p_fi);
+      const char *c_ct = g_file_info_get_content_type(p_fi);
+      if (c_ct != NULL) {
+         p_info->c_format = g_strdup(c_ct);
+      }
+      g_object_unref(p_fi);
+   } else {
+      g_clear_error(&p_err);
+   }
+
+   /* Dimensions via GdkPixbuf (without applying orientation). */
+   GdkPixbuf *p_pix = gdk_pixbuf_new_from_file(c_path, &p_err);
+   if (p_pix != NULL) {
+      p_info->i_width  = gdk_pixbuf_get_width(p_pix);
+      p_info->i_height = gdk_pixbuf_get_height(p_pix);
+      g_object_unref(p_pix);
+   } else {
+      g_clear_error(&p_err);
+   }
+
+   /* EXIF via libexif. */
+   ExifData *p_exif = exif_data_new_from_file(c_path);
+   if (p_exif != NULL) {
+      p_info->c_camera   = _dup_camera(p_exif);
+      p_info->c_focal    = _dup_exif_value(p_exif, EXIF_TAG_FOCAL_LENGTH);
+      p_info->c_aperture = _dup_exif_value(p_exif, EXIF_TAG_FNUMBER);
+      p_info->c_shutter  = _dup_exif_value(p_exif, EXIF_TAG_EXPOSURE_TIME);
+      p_info->c_iso      = _dup_exif_value(p_exif, EXIF_TAG_ISO_SPEED_RATINGS);
+      p_info->c_datetime = _dup_exif_value(p_exif, EXIF_TAG_DATE_TIME_ORIGINAL);
+      p_info->i_orientation = _get_orientation(p_exif);
+      exif_data_unref(p_exif);
+   }
+
+   g_free(c_path);
+   return (p_info);
+}
+
+void
+info_delete(GgazeInfo *p_info) {
+   if (p_info == NULL) {
+      return;
+   }
+   g_free(p_info->c_format);
+   g_free(p_info->c_camera);
+   g_free(p_info->c_lens);
+   g_free(p_info->c_focal);
+   g_free(p_info->c_aperture);
+   g_free(p_info->c_shutter);
+   g_free(p_info->c_iso);
+   g_free(p_info->c_datetime);
+   g_free(p_info);
+}
+
+static char *
+_join(GString *p_str, const char *c_label, const char *c_val) {
+   if (c_val != NULL && c_val[0] != '\0') {
+      g_string_append_printf(p_str, "%s: %s\n", c_label, c_val);
+   }
+   return (NULL);
+}
+
+char *
+info_format(const GgazeInfo *p_info) {
+   if (p_info == NULL) {
+      return (g_strdup(""));
+   }
+   GString *p_str = g_string_new(NULL);
+   g_string_append_printf(p_str, "%d×%d\n", p_info->i_width, p_info->i_height);
+   _join(p_str, "Format", p_info->c_format);
+   if (p_info->i_size > 0) {
+      char *c_sz = g_format_size(p_info->i_size);
+      g_string_append_printf(p_str, "Size: %s\n", c_sz);
+      g_free(c_sz);
+   }
+   _join(p_str, "Camera", p_info->c_camera);
+   _join(p_str, "Lens", p_info->c_lens);
+   _join(p_str, "Focal", p_info->c_focal);
+   _join(p_str, "Aperture", p_info->c_aperture);
+   _join(p_str, "Shutter", p_info->c_shutter);
+   _join(p_str, "ISO", p_info->c_iso);
+   _join(p_str, "Date", p_info->c_datetime);
+   if (p_info->i_orientation > 0) {
+      g_string_append_printf(p_str, "Orientation: %d\n", p_info->i_orientation);
+   }
+   /* trim trailing newline */
+   if (p_str->len > 0 && p_str->str[p_str->len - 1] == '\n') {
+      g_string_truncate(p_str, p_str->len - 1);
+   }
+   return (g_string_free(p_str, FALSE));
+}
\ No newline at end of file
diff --git a/src/info.h b/src/info.h
new file mode 100644
index 0000000..7680b72
--- /dev/null
+++ b/src/info.h
@@ -0,0 +1,45 @@
+#ifndef GGAZE_INFO_H
+#define GGAZE_INFO_H
+
+/*:*
+ * ggaze — image info / EXIF gather
+ *
+ * Gathers file info (dimensions, format, size) + EXIF (camera, lens, focal,
+ * aperture, shutter, ISO, datetime, orientation) into a GgazeInfo struct.
+ * Plain-C, no GtkWidget; uses GdkPixbuf (dims) + libexif (EXIF). Unit-testable.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include <gio/gio.h>
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+typedef struct {
+   int    i_width;
+   int    i_height;
+   char  *c_format;      /* content type (owned) */
+   gint64 i_size;        /* file size in bytes */
+   char  *c_camera;      /* Make + Model (owned) */
+   char  *c_lens;        /* Lens model (owned, or NULL) */
+   char  *c_focal;       /* FocalLength (owned) */
+   char  *c_aperture;    /* FNumber (owned) */
+   char  *c_shutter;     /* ExposureTime (owned) */
+   char  *c_iso;         /* ISOSpeedRatings (owned) */
+   char  *c_datetime;    /* DateTimeOriginal (owned) */
+   int    i_orientation; /* EXIF Orientation (1-8, 0 if none) */
+} GgazeInfo;
+
+/* Gather info for p_file (synchronous). Returns a new GgazeInfo (caller owns,
+ * free with info_delete) or NULL on failure. */
+GgazeInfo *info_new(GFile *p_file);
+void       info_delete(GgazeInfo *p_info);
+
+/* Format the info as a multi-line string (caller frees). */
+char *info_format(const GgazeInfo *p_info);
+
+G_END_DECLS
+
+#endif /* GGAZE_INFO_H */
\ No newline at end of file
diff --git a/src/shortcuts.c b/src/shortcuts.c
index 6853490..a23cfe2 100644
--- a/src/shortcuts.c
+++ b/src/shortcuts.c
@@ -37,6 +37,10 @@ static const ShortcutEntry SHORTCUTS[] = {
    {GDK_KEY_equal, 0, "win.zoom-in"},
    {GDK_KEY_minus, 0, "win.zoom-out"},
    {GDK_KEY_underscore, 0, "win.zoom-out"},
+   {GDK_KEY_f, 0, "win.fullscreen"},
+   {GDK_KEY_S, GDK_SHIFT_MASK, "win.slideshow"},
+   {GDK_KEY_i, 0, "win.info"},
+   {GDK_KEY_Escape, 0, "win.back"},
 };
 
 void
diff --git a/src/window.c b/src/window.c
index 003e7bf..4c26ea7 100644
--- a/src/window.c
+++ b/src/window.c
@@ -18,6 +18,7 @@
 #include <gtk/gtk.h>
 
 #include "gridview.h"
+#include "info.h"
 #include "loader/loader.h"
 #include "navigator.h"
 #include "shortcuts.h"
@@ -38,15 +39,24 @@ struct _GgazeWindow {
    GtkWidget    *p_viewer;          /* GgazeViewer — the large view */
    GgazeGrid    *p_grid;      /* the thumbnail grid (the "grid" stack child) */
    int           i_grid_size; /* current thumbnail size (64-512, decision T) */
+   GtkWidget    *p_overlay; /* GtkOverlay wrapping the stack (for info label) */
+   GtkWidget    *p_info_lbl;  /* info overlay label (auto-hides) */
+   guint         u_info_hide; /* info auto-hide timeout id (0=none) */
+   guint         u_slideshow; /* slideshow timeout id (0=off) */
+   gboolean      b_fullscreen;
+   guint         u_hdr_hide; /* fullscreen header auto-hide timeout */
 };
 
 G_DEFINE_TYPE(GgazeWindow, ggaze_window, GTK_TYPE_APPLICATION_WINDOW)
 
 /* --- forward decls ------------------------------------------------------- */
-static void _load_current(GgazeWindow *p_win);
-static void _prefetch(GgazeWindow *p_win);
-static void _update_header(GgazeWindow *p_win);
-static void _on_grid_activate(GgazeGrid *p_grid, gpointer p_data);
+static void     _load_current(GgazeWindow *p_win);
+static void     _prefetch(GgazeWindow *p_win);
+static void     _update_header(GgazeWindow *p_win);
+static void     _on_grid_activate(GgazeGrid *p_grid, gpointer p_data);
+static void     _show_info(GgazeWindow *p_win);
+static void     _hide_info(GgazeWindow *p_win);
+static gboolean _slideshow_tick(gpointer p_data);
 
 /* --- actions ------------------------------------------------------------- */
 
@@ -269,6 +279,108 @@ _action_zoom_out(GSimpleAction *p_a, GVariant *p_v, gpointer p_data) {
    }
 }
 
+/* --- M4: fullscreen / slideshow / info / back --------------------------- */
+
+static void
+_action_fullscreen(GSimpleAction *p_a, GVariant *p_v, gpointer p_data) {
+   (void)p_a;
+   (void)p_v;
+   GgazeWindow *p_win = GGAZE_WINDOW(p_data);
+   if (p_win->b_fullscreen) {
+      gtk_window_unfullscreen(GTK_WINDOW(p_win));
+      p_win->b_fullscreen = FALSE;
+   } else {
+      gtk_window_fullscreen(GTK_WINDOW(p_win));
+      p_win->b_fullscreen = TRUE;
+   }
+}
+
+static void
+_action_slideshow(GSimpleAction *p_a, GVariant *p_v, gpointer p_data) {
+   (void)p_a;
+   (void)p_v;
+   GgazeWindow *p_win = GGAZE_WINDOW(p_data);
+   if (p_win->u_slideshow != 0) {
+      g_source_remove(p_win->u_slideshow);
+      p_win->u_slideshow = 0;
+   } else {
+      /* 3-second default; GSettings slideshow-delay wired in M10 */
+      p_win->u_slideshow = g_timeout_add_seconds(3, _slideshow_tick, p_win);
+   }
+}
+
+static void
+_action_info(GSimpleAction *p_a, GVariant *p_v, gpointer p_data) {
+   (void)p_a;
+   (void)p_v;
+   _show_info(GGAZE_WINDOW(p_data));
+}
+
+static void
+_action_back(GSimpleAction *p_a, GVariant *p_v, gpointer p_data) {
+   (void)p_a;
+   (void)p_v;
+   GgazeWindow *p_win = GGAZE_WINDOW(p_data);
+   if (p_win->b_fullscreen) {
+      gtk_window_unfullscreen(GTK_WINDOW(p_win));
+      p_win->b_fullscreen = FALSE;
+   } else {
+      const char *c_cur =
+         gtk_stack_get_visible_child_name(GTK_STACK(p_win->p_stack));
+      if (g_strcmp0(c_cur, "large") == 0) {
+         gtk_stack_set_visible_child_name(GTK_STACK(p_win->p_stack), "grid");
+      } else {
+         gtk_window_close(GTK_WINDOW(p_win));
+      }
+   }
+}
+
+static gboolean
+_slideshow_tick(gpointer p_data) {
+   GgazeWindow *p_win = GGAZE_WINDOW(p_data);
+   if (p_win->p_nav != NULL) {
+      navigator_next(p_win->p_nav);
+   }
+   return (G_SOURCE_CONTINUE);
+}
+
+static gboolean
+_info_hide_tick(gpointer p_data) {
+   GgazeWindow *p_win = GGAZE_WINDOW(p_data);
+   _hide_info(p_win);
+   p_win->u_info_hide = 0;
+   return (G_SOURCE_REMOVE);
+}
+
+static void
+_show_info(GgazeWindow *p_win) {
+   if (p_win->p_nav == NULL) {
+      return;
+   }
+   GFile *p_cur = navigator_get_current(p_win->p_nav);
+   if (p_cur == NULL) {
+      return;
+   }
+   GgazeInfo *p_info = info_new(p_cur);
+   if (p_info == NULL) {
+      return;
+   }
+   char *c_text = info_format(p_info);
+   gtk_label_set_text(GTK_LABEL(p_win->p_info_lbl), c_text);
+   g_free(c_text);
+   info_delete(p_info);
+   gtk_widget_set_visible(p_win->p_info_lbl, TRUE);
+   if (p_win->u_info_hide != 0) {
+      g_source_remove(p_win->u_info_hide);
+   }
+   p_win->u_info_hide = g_timeout_add_seconds(5, _info_hide_tick, p_win);
+}
+
+static void
+_hide_info(GgazeWindow *p_win) {
+   gtk_widget_set_visible(p_win->p_info_lbl, FALSE);
+}
+
 static const GActionEntry ACTIONS[] = {
    {.name = "prev", .activate = _action_prev},
    {.name = "next", .activate = _action_next},
@@ -282,6 +394,10 @@ static const GActionEntry ACTIONS[] = {
    {.name = "toggle-view", .activate = _action_toggle_view},
    {.name = "zoom-in", .activate = _action_zoom_in},
    {.name = "zoom-out", .activate = _action_zoom_out},
+   {.name = "fullscreen", .activate = _action_fullscreen},
+   {.name = "slideshow", .activate = _action_slideshow},
+   {.name = "info", .activate = _action_info},
+   {.name = "back", .activate = _action_back},
 };
 
 /* --- drop target --------------------------------------------------------- */
@@ -485,6 +601,18 @@ ggaze_window_dispose(GObject *p_obj) {
    g_clear_object(&p_win->p_prefetch_cancel);
    g_cancellable_cancel(p_win->p_cancel);
    g_clear_object(&p_win->p_cancel);
+   if (p_win->u_slideshow != 0) {
+      g_source_remove(p_win->u_slideshow);
+      p_win->u_slideshow = 0;
+   }
+   if (p_win->u_info_hide != 0) {
+      g_source_remove(p_win->u_info_hide);
+      p_win->u_info_hide = 0;
+   }
+   if (p_win->u_hdr_hide != 0) {
+      g_source_remove(p_win->u_hdr_hide);
+      p_win->u_hdr_hide = 0;
+   }
    g_clear_pointer(&p_win->p_cache, texturecache_delete);
    g_clear_pointer(&p_win->p_trash, trash_delete);
    g_clear_pointer(&p_win->p_thumb, thumbnail_delete);
@@ -517,7 +645,17 @@ ggaze_window_init(GgazeWindow *p_win) {
    p_win->p_stack = gtk_stack_new();
    gtk_stack_set_transition_type(GTK_STACK(p_win->p_stack),
                                  GTK_STACK_TRANSITION_TYPE_CROSSFADE);
-   gtk_window_set_child(GTK_WINDOW(p_win), p_win->p_stack);
+
+   /* Wrap the stack in a GtkOverlay so the info label can float on top. */
+   p_win->p_overlay = gtk_overlay_new();
+   gtk_overlay_set_child(GTK_OVERLAY(p_win->p_overlay), p_win->p_stack);
+   p_win->p_info_lbl = gtk_label_new("");
+   gtk_widget_add_css_class(p_win->p_info_lbl, "ggaze-info");
+   gtk_widget_set_margin_start(p_win->p_info_lbl, 12);
+   gtk_widget_set_margin_top(p_win->p_info_lbl, 12);
+   gtk_widget_set_visible(p_win->p_info_lbl, FALSE);
+   gtk_overlay_add_overlay(GTK_OVERLAY(p_win->p_overlay), p_win->p_info_lbl);
+   gtk_window_set_child(GTK_WINDOW(p_win), p_win->p_overlay);
 
    GtkWidget *p_grid = gtk_label_new("grid");
    gtk_widget_add_css_class(p_grid, "dim-label");
@@ -642,4 +780,10 @@ ggaze_window_last(GgazeWindow *p_win) {
    if (p_win->p_nav != NULL) {
       navigator_last(p_win->p_nav);
    }
-}
\ No newline at end of file
+}
+
+GtkStack *
+ggaze_window_get_stack(GgazeWindow *p_win) {
+   g_return_val_if_fail(GGAZE_IS_WINDOW(p_win), NULL);
+   return (GTK_STACK(p_win->p_stack));
+}
diff --git a/src/window.h b/src/window.h
index 10612db..43572d0 100644
--- a/src/window.h
+++ b/src/window.h
@@ -32,6 +32,10 @@ GgazeWindow *ggaze_window_new(GgazeApp *p_app);
  * current image into the viewer and switches the stack to "large". */
 void ggaze_window_open(GgazeWindow *p_win, GFile *p_arg);
 
+/* The GtkStack (grid/large) — the tests use this instead of
+ * gtk_window_get_child (which now returns the wrapping GtkOverlay). */
+GtkStack *ggaze_window_get_stack(GgazeWindow *p_win);
+
 /* Navigation over the current folder (bound to h/l/Left/Right/g/G via
  * shortcuts.c). No-ops if nothing is open. */
 void ggaze_window_prev(GgazeWindow *p_win);