gtk: add read-only indicator for surfaces

Fixes: #9889

src/apprt/gtk/class/application.zig

@@ -733,6 +733,7 @@ pub const Application = extern struct {
             .toggle_split_zoom => return Action.toggleSplitZoom(target),
             .show_on_screen_keyboard => return Action.showOnScreenKeyboard(target),
             .command_finished => return Action.commandFinished(target, value),
+            .readonly => return Action.setReadonly(target, value),
 
             .start_search => Action.startSearch(target, value),
             .end_search => Action.endSearch(target),
@@ -753,7 +754,6 @@ pub const Application = extern struct {
             .check_for_updates,
             .undo,
             .redo,
-            .readonly,
             => {
                 log.warn("unimplemented action={}", .{action});
                 return false;
@@ -2678,6 +2678,15 @@ const Action = struct {
         }
     }
 
+    pub fn setReadonly(target: apprt.Target, value: apprt.Action.Value(.readonly)) bool {
+        switch (target) {
+            .app => return false,
+            .surface => |surface| {
+                return surface.rt_surface.gobj().setReadonly(value);
+            },
+        }
+    }
+
     pub fn keySequence(target: apprt.Target, value: apprt.Action.Value(.key_sequence)) bool {
         switch (target) {
             .app => {

src/apprt/gtk/class/surface.zig

@@ -400,6 +400,25 @@ pub const Surface = extern struct {
                 },
             );
         };
+
+        pub const readonly = struct {
+            pub const name = "readonly";
+            const impl = gobject.ext.defineProperty(
+                name,
+                Self,
+                bool,
+                .{
+                    .default = false,
+                    .accessor = gobject.ext.typedAccessor(
+                        Self,
+                        bool,
+                        .{
+                            .getter = getReadonly,
+                        },
+                    ),
+                },
+            );
+        };
     };
 
     pub const signals = struct {
@@ -1106,6 +1125,20 @@ pub const Surface = extern struct {
         return true;
     }
 
+    /// Get the readonly state from the core surface.
+    pub fn getReadonly(self: *Self) bool {
+        const priv: *Private = self.private();
+        const surface = priv.core_surface orelse return false;
+        return surface.readonly;
+    }
+
+    /// Notify anyone interested that the readonly status has changed.
+    pub fn setReadonly(self: *Self, _: apprt.Action.Value(.readonly)) bool {
+        self.as(gobject.Object).notifyByPspec(properties.readonly.impl.param_spec);
+
+        return true;
+    }
+
     /// Key press event (press or release).
     ///
     /// At a high level, we want to construct an `input.KeyEvent` and
@@ -3480,6 +3513,7 @@ pub const Surface = extern struct {
                 properties.@"title-override".impl,
                 properties.zoom.impl,
                 properties.@"is-split".impl,
+                properties.readonly.impl,
 
                 // For Gtk.Scrollable
                 properties.hadjustment.impl,

src/apprt/gtk/css/style.css

@@ -134,6 +134,16 @@ label.resize-overlay {
   border-style: solid;
 }
 
+.surface .readonly_overlay {
+  /* Should be the equivalent of the following SwiftUI color: */
+  /* Color(hue: 0.08, saturation: 0.5, brightness: 0.8) */
+  color: hsl(25 50 75);
+  padding: 8px 8px 8px 8px;
+  margin: 8px 8px 8px 8px;
+  border-radius: 6px 6px 6px 6px;
+  outline-style: solid;
+  outline-width: 1px;
+}
 /*
  * Command Palette
  */

src/apprt/gtk/ui/1.2/surface.blp

@@ -71,6 +71,39 @@ Overlay terminal_page {
     }
   };
 
+  [overlay]
+  Revealer {
+    reveal-child: bind template.readonly;
+    transition-type: crossfade;
+    transition-duration: 500;
+    // Revealers take up the full size, we need this to not capture events.
+    can-focus: false;
+    can-target: false;
+    focusable: false;
+
+    Box readonly_overlay {
+      styles [
+        "readonly_overlay",
+      ]
+
+      // TODO: the tooltip doesn't actually work, but keep it here for now so
+      // that we can get the tooltip text translated.
+      has-tooltip: true;
+      tooltip-text: _("This terminal is in read-only mode. You can still view, select, and scroll through the content, but no input events will be sent to the running application.");
+      halign: end;
+      valign: start;
+      spacing: 6;
+
+      Image {
+        icon-name: "changes-prevent-symbolic";
+      }
+
+      Label {
+        label: _("Read-only");
+      }
+    }
+  }
+
   [overlay]
   ProgressBar progress_bar_overlay {
     styles [