review: say why a read-only signature field can't be signed

crates/ui-egui/src/tools/sign_ui/panel.rs

@@ -120,8 +120,20 @@ pub(crate) fn panel(ui: &mut egui::Ui, t: &Tokens, sigs: &[SignatureInfo], expan
         egui::Frame::NONE.inner_margin(egui::Margin { left: 44, right: 4, top: 2, bottom: 8 }).show(ui, |ui| {
             ui.spacing_mut().item_spacing.y = 3.0;
             if !s.signed {
-                // The writer refuses a read-only field, so the panel does not offer to sign it.
-                if !s.read_only && widgets::pill_button(ui, tl!("Sign this field…"), true).clicked() {
+                // The writer refuses a read-only field, so the panel does not offer to sign it,
+                // and says why instead of showing an empty expansion.
+                if s.read_only {
+                    ui.add(
+                        egui::Label::new(
+                            egui::RichText::new(tl!("This field is read-only, so it can't be signed."))
+                                .font(theme::regular(11.5))
+                                .color(t.text_muted),
+                        )
+                        .wrap(),
+                    );
+                    return;
+                }
+                if widgets::pill_button(ui, tl!("Sign this field…"), true).clicked() {
                     action = Some(PanelAction::Sign(s.field.clone()));
                 }
                 return;