Removing slopes no longer leaves unowned seal cells; loading clears them

A slope's seal can sit in a cell its neighbour leaves open (a slope's
open corner, or under a grate, screw or screen), but removal only
cleared it when the neighbour was a sloped wall, so cutting a sloped
belt next to a sloped filter or grate could leave a one-cell belt that
nothing owned and nothing could remove. Removal now asks whether the
neighbour fills that cell itself, and loading a world clears belt and
screen cells that no block or seal owns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/agent-log.md

@@ -4,6 +4,17 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: Unremovable one-cell belt left after cutting slopes (Claude Code)
+
+- The owner's exported save had a lone belt cell no block owned, so nothing could build there or
+  remove it. It was a sloped belt's seal: a seal may sit in a cell its neighbour leaves open (a
+  slope's open corner, or under a grate, screw or screen), but removal only let it go when the
+  neighbour was a sloped wall. Now `blockOwnsCell` decides (`src/sim/blocks.js`), and loading a
+  world clears belt and screen cells no block or seal owns (`clearStraySeals`, `save.js`).
+- Found with a fuzz of random slope/belt/filter/wall/grate builds removed in random orders and by
+  cut (0 strays in 4000 runs after the fix). The owner's save loads clean and the spot is
+  buildable. Tests in `t_slope_belts.js` fail on the old code. `run_all --browser`: ALL PASSED.
+
 ## 2026-10-27: t42 feedback: downhill belts drop the sidewall too; new tasks (Claude Code)
 
 - Downhill sloped belts now draw as plain belts: no far wall and no ribs, placed or as ghosts or

src/sim/blocks.js

@@ -27,6 +27,22 @@ function slopeSealOwner(i) {
   }
   return -1;
 }
+// Does block b's own piece fill cell i (with a part, or else with a material)? A slope's seal can
+// sit in a cell its neighbour leaves open: a slope's open corner, or under a grate, screw or screen.
+function blockOwnsCell(b, i, asPart) {
+  const c = blockP[b];
+  if (!c || ((isSlope(c) || isSlopeBelt(c)) && !slopeCell(c, (i % W) & 3, ((i / W) | 0) & 3))) return false;
+  return asPart ? isFilter(c) : !(isFilter(c) || c === BP_GRATE || c === BP_SCREW || c === BP_PIPE);
+}
+// Belt and screen cells that a removed slope left behind in an empty block (a removal bug in
+// older worlds): nothing owns them, so nothing could build there or take them away.
+function clearStraySeals() {
+  for (let i = 0; i < N; i++) {
+    if (mat[i] !== BELTL && mat[i] !== BELTR && part[i] !== P_FILTER) continue;
+    if (blockP[(((i / W) | 0) >> 2) * BW + ((i % W) >> 2)] || slopeSealOwner(i) >= 0) continue;
+    if (part[i] === P_FILTER) { part[i] = 0; wakeIdx(i); } else { setCell(i, EMPTY, temp[i], 0); markPow(i, false); }
+  }
+}
 function wallCells(code, bx, by) {
   return blockCellList(bx, by).filter(i => !(isSlope(code) || isSlopeBelt(code)) || slopeCell(code, (i % W) & 3, ((i / W) | 0) & 3));
 }
@@ -158,8 +174,8 @@ function removeBlock(bx, by) {
   if (isSlope(code) || isSlopeBelt(code)) {
     const i = slopeSeal(code, bx, by);
     if (i >= 0) {
-      const nb = (((i / W) | 0) >> 2) * BW + ((i % W) >> 2), nc = blockP[nb];
-      if ((!nc || (isSlope(nc) && !slopeCell(nc, (i % W) & 3, ((i / W) | 0) & 3))) && (mat[i] === (isBelt(code) ? (v ? BELTR : BELTL) : WALL_OF[v]) || (isFilter(code) && part[i] === P_FILTER)) && !(flags[i] & F_NAT)) { if (isFilter(code)) { part[i] = 0; wakeIdx(i); } else { setCell(i, EMPTY, temp[i], 0); if (isBelt(code)) markPow(i, false); } }
+      const nb = (((i / W) | 0) >> 2) * BW + ((i % W) >> 2);
+      if (!blockOwnsCell(nb, i, isFilter(code)) && (mat[i] === (isBelt(code) ? (v ? BELTR : BELTL) : WALL_OF[v]) || (isFilter(code) && part[i] === P_FILTER)) && !(flags[i] & F_NAT)) { if (isFilter(code)) { part[i] = 0; wakeIdx(i); } else { setCell(i, EMPTY, temp[i], 0); if (isBelt(code)) markPow(i, false); } }
       cleared.push(i);
     }
   }

src/sim/save.js

@@ -119,6 +119,7 @@ function deserializeWorld(meta, buffers) {
   seen = new Uint8Array(NMAT); meta.seen.forEach((v, k) => { if (k < NMAT) seen[k] = v; });
   ledger = meta.ledger || {};
   Object.assign(stats, meta.stats || {});
+  clearStraySeals();
   powerDirty = true; rebuildPower(); slopeCleatsTick(false); updateRoof();
   if (!weather.base) weather.base = worldWater(); // older saves: take the water it has now as its starting amount
   if (!meta.specks) clearHardSpecks(true); // older saves: tidy away tiny specks of too-hard rock once

test/t_slope_belts.js

@@ -238,4 +238,17 @@ const legacy=JSON.parse(require('zlib').gunzipSync(fs.readFileSync(__dirname+'/f
 globalThis.legacyBeltSave=legacy;
 run('deserializeWorld(legacyBeltSave.meta,legacyBeltSave.buffers)');
 check('old v2 saves keep all wall slope ids and materials',run('Array.from({length:4},(_,k)=>{const b=6*BW+2+k*3;return blockP[b]===15+k && blockV[b]===RSTONE && blockIntegrity(b)===10;}).every(Boolean)'));
+// A slope's seal can sit in the open corner of a neighbouring slope or under a grate or screen;
+// taking both away must leave nothing behind (it used to leave an unremovable belt cell).
+const strayCells="(()=>{let n=0;for(let i=0;i<N;i++){const m=mat[i];if((m===BELTL||m===BELTR||part[i]===P_FILTER||m===RSTONE)&&!blockP[((i/W|0)>>2)*BW+((i%W)>>2)]&&slopeSealOwner(i)<0)n++;}return n;})()";
+for(const [first,second] of [['BP_FILTER_DOWN,128,10,12','BP_BELT_DOWN,0,10,11'],['BP_GRATE,0,10,12','BP_BELT_DOWN,0,10,11'],['BP_FILTER,0,10,12','BP_SLOPE_BL,RSTONE,10,11']]) for(const order of [0,1]) for(const built of [0,1]) {
+  fresh();
+  const [a,b]=built?[second,first]:[first,second];
+  run(`placeBlock(${a},true,{m:[],inv:0});placeBlock(${b},true,{m:[],inv:0});`);
+  run(order?'removeBlock(10,12);removeBlock(10,11);':'removeBlock(10,11);removeBlock(10,12);');
+  check(`removing ${first.split(',')[0]} and the ${second.split(',')[0]} seal inside it (built ${built}, removed ${order}) leaves no stray cell`,run(strayCells)===0,run(strayCells));
+}
+fresh();
+run("setCell(40*W+43,BELTL,20,0);globalThis.straySave=serializeWorld();deserializeWorld(straySave.meta,straySave.buffers);");
+check('loading a world clears a stray belt cell left by the old removal bug',run('mat[40*W+43]===EMPTY'));
 done();