Fix 3d: stroke vanished on commit (establish full buffer on first execute)

The brush commits via a GPU-canvas readback and relies on the action's first
execute() to SET raw_pixels from that readback — at that point raw_pixels is empty
(new keyframe) or the pre-stroke state, never something a dirty-rect diff can stamp
onto. My initial diff-only execute skipped (to avoid corruption), so the stroke
disappeared.

Fix: the action keeps the full post-edit buffer ONLY for the first execute (the
commit), assigning raw_pixels outright exactly like the old code; it's taken/dropped
immediately, so the action sitting in the undo stack still retains just the small
diff. Redo replays via the diff onto the now-resident base.

Also harden the diff itself for the blank-base case: `before_blank` lets apply_after
build from a transparent buffer (redo of a first stroke after undo-to-blank) and
apply_before restore to empty; the resident-base skip is kept only for non-blank
bases (faulted in before undo/redo). Tests cover commit/redo from empty.

lightningbeam-ui/lightningbeam-core/src/actions/raster_diff.rs

@@ -31,6 +31,11 @@ pub struct RasterDiff {
     before_region: Vec<u8>,
     /// bbox-sized RGBA (`w*h*4`) of the region after the edit.
     after_region: Vec<u8>,
+    /// The pre-edit buffer was blank (empty/unallocated) — i.e. this was the first
+    /// edit on a fresh keyframe. Lets `apply_after` build from a transparent base
+    /// (the commit/redo path often starts with empty `raw_pixels`) and `apply_before`
+    /// restore to blank, instead of requiring a resident base.
+    before_blank: bool,
 }
 
 impl RasterDiff {
@@ -39,6 +44,7 @@ impl RasterDiff {
     /// keyframe's first stroke), treated as fully transparent.
     pub fn compute(before: &[u8], after: &[u8], width: u32, height: u32) -> Self {
         let n = width as usize * height as usize * 4;
+        let before_blank = before.len() != n;
         // Normalize both sides to full length; empty/short ⇒ transparent.
         let before_full = normalize(before, n);
         let after_full = normalize(after, n);
@@ -63,7 +69,7 @@ impl RasterDiff {
 
         if !any {
             return Self { full_width: width, full_height: height, bbox: None,
-                          before_region: Vec::new(), after_region: Vec::new() };
+                          before_region: Vec::new(), after_region: Vec::new(), before_blank };
         }
 
         let bw = max_x - min_x + 1;
@@ -83,6 +89,7 @@ impl RasterDiff {
             bbox: Some((min_x as u32, min_y as u32, bw as u32, bh as u32)),
             before_region: crop(&before_full),
             after_region: crop(&after_full),
+            before_blank,
         }
     }
 
@@ -91,25 +98,45 @@ impl RasterDiff {
         self.before_region.len() + self.after_region.len()
     }
 
-    /// Restore the pre-edit pixels into `raw` (undo).
+    /// Restore the pre-edit pixels into `raw` (undo / first-execute rollback).
     pub fn apply_before(&self, raw: &mut Vec<u8>) {
-        self.apply(&self.before_region, raw);
+        if self.bbox.is_none() {
+            return; // no change
+        }
+        if self.before_blank {
+            // The frame was blank before this edit (it was the first stroke); undoing
+            // it returns to blank regardless of the current buffer.
+            raw.clear();
+            return;
+        }
+        self.stamp_resident(&self.before_region, raw);
     }
 
-    /// Re-apply the post-edit pixels into `raw` (redo).
+    /// Apply the post-edit pixels into `raw` (commit / redo).
     pub fn apply_after(&self, raw: &mut Vec<u8>) {
-        self.apply(&self.after_region, raw);
+        if self.bbox.is_none() {
+            return; // no change
+        }
+        if self.before_blank {
+            // Base was blank: build a full transparent buffer then stamp the bbox. The
+            // commit/redo path frequently starts from empty `raw_pixels` here.
+            let n = self.full_width as usize * self.full_height as usize * 4;
+            raw.clear();
+            raw.resize(n, 0);
+        }
+        self.stamp_resident(&self.after_region, raw);
     }
 
-    fn apply(&self, region: &[u8], raw: &mut Vec<u8>) {
+    /// Stamp a bbox-sized region into `raw`, which must already be full-size. If it
+    /// isn't (a non-blank base that the editor failed to fault in), skip rather than
+    /// resize-and-corrupt — the frame will re-page to its container state.
+    fn stamp_resident(&self, region: &[u8], raw: &mut [u8]) {
         let n = self.full_width as usize * self.full_height as usize * 4;
         let (x, y, bw, bh) = match self.bbox {
             Some(b) => b,
-            None => return, // no change
+            None => return,
         };
         if raw.len() != n {
-            // Base not resident: the editor should have faulted it in. Skip rather
-            // than resize-and-corrupt — the frame will re-page to its container state.
             eprintln!(
                 "⚠️ [RASTER_DIFF] base not resident ({} != {}); skipping undo/redo apply",
                 raw.len(), n
@@ -157,17 +184,27 @@ mod tests {
     #[test]
     fn blank_before_first_stroke() {
         let (w, h) = (4, 4);
+        let n = (w * h * 4) as usize;
         let before: Vec<u8> = Vec::new(); // blank keyframe
-        let mut after = vec![0u8; (w * h * 4) as usize];
+        let mut after = vec![0u8; n];
         let i = ((1 * w + 1) * 4) as usize;
         after[i..i + 4].copy_from_slice(&[255, 0, 0, 255]);
         let diff = RasterDiff::compute(&before, &after, w, h);
         assert_eq!(diff.bbox, Some((1, 1, 1, 1)));
 
-        // Undo onto the resident post-stroke buffer → fully transparent (blank-equiv).
+        // First execute / redo from EMPTY raw_pixels (the real commit path): builds
-        let mut buf = after.clone();
+        // the full buffer from transparent + the stroke.
+        let mut buf: Vec<u8> = Vec::new();
+        diff.apply_after(&mut buf);
+        assert_eq!(buf, after, "commit/redo must build the frame from a blank base");
+
+        // Undo the first stroke → back to blank (empty).
         diff.apply_before(&mut buf);
-        assert_eq!(buf, vec![0u8; (w * h * 4) as usize]);
+        assert!(buf.is_empty(), "undoing the first stroke restores the blank keyframe");
+
+        // Redo again from the now-empty buffer.
+        diff.apply_after(&mut buf);
+        assert_eq!(buf, after);
     }
 
     #[test]

lightningbeam-ui/lightningbeam-core/src/actions/raster_fill.rs

@@ -13,6 +13,9 @@ pub struct RasterFillAction {
     height: u32,
     name: String,
     diff: RasterDiff,
+    /// Full post-fill buffer, kept only for the first `execute` (commit); see
+    /// `RasterStrokeAction::full_after`.
+    full_after: Option<Vec<u8>>,
 }
 
 impl RasterFillAction {
@@ -25,7 +28,8 @@ impl RasterFillAction {
         height: u32,
     ) -> Self {
         let diff = RasterDiff::compute(&buffer_before, &buffer_after, width, height);
-        Self { layer_id, time, width, height, name: "Flood fill".to_string(), diff }
+        Self { layer_id, time, width, height, name: "Flood fill".to_string(),
+               diff, full_after: Some(buffer_after) }
     }
 
     pub fn with_description(mut self, name: &str) -> Self {
@@ -43,7 +47,11 @@ impl Action for RasterFillAction {
             _ => return Err("Not a raster layer".to_string()),
         };
         let kf = raster.ensure_keyframe_at(self.time, self.width, self.height);
-        self.diff.apply_after(&mut kf.raw_pixels);
+        if let Some(full) = self.full_after.take() {
+            kf.raw_pixels = full;
+        } else {
+            self.diff.apply_after(&mut kf.raw_pixels);
+        }
         kf.texture_dirty = true;
         kf.dirty = true;
         Ok(())

lightningbeam-ui/lightningbeam-core/src/actions/raster_stroke.rs

@@ -25,6 +25,12 @@ pub struct RasterStrokeAction {
     width: u32,
     height: u32,
     diff: RasterDiff,
+    /// The full post-stroke buffer, kept ONLY for the first `execute` (the commit),
+    /// which establishes `raw_pixels` exactly like the old code did — robust no matter
+    /// what state the working buffer is in (empty new keyframe, GPU-canvas readback,
+    /// etc.). Taken (dropped) on first execute, so the action sitting in the undo stack
+    /// retains only the small `diff`; redo then replays via the diff.
+    full_after: Option<Vec<u8>>,
 }
 
 impl RasterStrokeAction {
@@ -43,14 +49,20 @@ impl RasterStrokeAction {
         height: u32,
     ) -> Self {
         let diff = RasterDiff::compute(&buffer_before, &buffer_after, width, height);
-        Self { layer_id, time, width, height, diff }
+        Self { layer_id, time, width, height, diff, full_after: Some(buffer_after) }
     }
 }
 
 impl Action for RasterStrokeAction {
     fn execute(&mut self, document: &mut Document) -> Result<(), String> {
         let kf = get_keyframe_mut(document, &self.layer_id, self.time, self.width, self.height)?;
-        self.diff.apply_after(&mut kf.raw_pixels);
+        if let Some(full) = self.full_after.take() {
+            // First execute (commit): assign the full buffer outright.
+            kf.raw_pixels = full;
+        } else {
+            // Redo: replay via the diff onto the (resident) base.
+            self.diff.apply_after(&mut kf.raw_pixels);
+        }
         kf.texture_dirty = true;
         kf.dirty = true;
         Ok(())