Format: Slide background fill applies to shapes and text boxes only

shape.fill {background: true} leaves the fill of pictures, connectors,
tables and charts unchanged. Only a shape or text box (p:sp) saves
useBgFill in a .pptx. With a group selected, its shapes and text boxes
take the fill and the group's own fill is cleared. When no selected
object can take the fill, the command returns an error.

The Format pane leaves Slide background fill out of a picture's Fill
options, and shows it chosen for a group whose shapes and text boxes
all have it.

crates/engine/src/cmd/shape.rs

@@ -90,7 +90,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Shape Fill",
             ["Shape Format", "Shape Styles"],
             None,
-            "{color? | none?: true | background?: true (the slide background shows through) | gradient?: {stops: [[pos, color]], angle?, kind?: linear|radial} | picture?: base64 | pattern?: {preset, fg, bg} | transparency?: 0..1 | reset?: true, ids?}; pictureProps?: {tile?: bool, offsetX?, offsetY?: pt, scaleX?, scaleY?: 0..10, align?: tl|t|tr|l|ctr|r|bl|b|br, mirror?: none|x|y|xy, stretch?: [l, t, r, b] fractions, rotateWithShape?: bool} adjusts a picture fill; a group also formats its members",
+            "{color? | none?: true | background?: true (the slide background shows through; shapes and text boxes only) | gradient?: {stops: [[pos, color]], angle?, kind?: linear|radial} | picture?: base64 | pattern?: {preset, fg, bg} | transparency?: 0..1 | reset?: true, ids?}; pictureProps?: {tile?: bool, offsetX?, offsetY?: pt, scaleX?, scaleY?: 0..10, align?: tl|t|tr|l|ctr|r|bl|b|br, mirror?: none|x|y|xy, stretch?: [l, t, r, b] fractions, rotateWithShape?: bool} adjusts a picture fill; a group also formats its members",
             has_selection,
             fill
         ),
@@ -612,7 +612,27 @@ fn fill(s: &mut Session, p: &Value) -> Result<Value> {
         }
         fills
     };
+    // Only a shape or text box (`p:sp`) saves a slide background fill (`useBgFill`). Pictures,
+    // connectors, tables and charts keep their fill, and a group's own fill is cleared.
+    let background = matches!(new_fill, Some(Some(Fill::Background)));
+    if background {
+        let mut any = false;
+        for id in targets(s, p)? {
+            if let Some(sh) = st.shape(id) {
+                deckcraft_model::walk(std::slice::from_ref(sh), &mut |sh, _| any |= matches!(sh.kind, deckcraft_model::ShapeKind::Shape));
+            }
+        }
+        if !any {
+            return Err(bad("shape.fill", "slide background fill applies to shapes and text boxes"));
+        }
+    }
     edit_group_appearance(s, p, "shape.fill", |sh| {
+        if background && !matches!(sh.kind, deckcraft_model::ShapeKind::Shape) {
+            if matches!(sh.kind, deckcraft_model::ShapeKind::Group { .. }) {
+                sh.fill = None;
+            }
+            return Ok(());
+        }
         if let Some(f) = &new_fill {
             sh.fill.clone_from(f);
         }
@@ -1645,6 +1665,39 @@ mod tests {
         assert_eq!(filled, 1, "the shape keeps the background fill after a save and reopen");
     }
 
+    /// A picture cannot save a slide background fill, so `background` leaves its fill alone.
+    #[test]
+    fn background_fill_skips_pictures() {
+        let mut s = Session::with_new();
+        s.execute("slide.new", &json!({"layout": "blank"})).unwrap();
+        let shape = s.execute("shape.insert", &json!({"preset": "rect", "rect": [100, 100, 200, 100]})).unwrap()["id"].as_u64().unwrap();
+        let png = deckcraft_render::Image { width: 2, height: 2, pixels: [255, 0, 0, 255].repeat(4) }.to_png();
+        let pic = s
+            .execute("insert.picture", &json!({"data": crate::cmd::base64_encode(&png), "name": "red.png", "rect": [400, 100, 100, 100]}))
+            .unwrap()["id"]
+            .as_u64()
+            .unwrap();
+        let fill = |s: &Session, id: u64| s.doc().unwrap().shape(ShapeId(id as u32)).unwrap().fill.clone();
+        let err = s.execute("shape.fill", &json!({"background": true, "ids": [pic]})).unwrap_err();
+        assert!(err.to_string().contains("shapes and text boxes"), "{err}");
+        assert_eq!(fill(&s, pic), None);
+
+        s.execute("edit.select", &json!({"ids": [shape, pic]})).unwrap();
+        let group = s.execute("arrange.group", &json!({})).unwrap()["id"].as_u64().unwrap();
+        s.execute("shape.fill", &json!({"color": "#ff0000"})).unwrap();
+        s.execute("shape.fill", &json!({"background": true})).unwrap();
+        assert_eq!(fill(&s, shape), Some(Fill::Background));
+        assert!(matches!(fill(&s, pic), Some(Fill::Solid { .. })), "the picture keeps its fill");
+        assert_eq!(fill(&s, group), None, "the group's own fill is cleared");
+        let bytes = deckcraft_pptx::export(&s.doc().unwrap().doc).unwrap();
+        let back = deckcraft_pptx::import(&bytes).unwrap();
+        let mut filled = 0;
+        for slide in &back.slides {
+            deckcraft_model::walk(&slide.shapes, &mut |sh, _| filled += usize::from(sh.fill == Some(Fill::Background)));
+        }
+        assert_eq!(filled, 1, "the shape keeps the background fill after a save and reopen");
+    }
+
     fn rotation(s: &Session) -> f64 {
         let st = s.active().unwrap();
         let id = st.selection.shapes[0];