emit remove before linking up sibling tiles

otherwise emitting the update from updatePreviousSibling has
the wrong idx

src/domain/session/room/timeline/TilesCollection.js

@@ -195,11 +195,14 @@ export class TilesCollection extends BaseObservableList {
     _removeTile(tileIdx, tile) {
         const prevTile = this._getTileAtIdx(tileIdx - 1);
         const nextTile = this._getTileAtIdx(tileIdx + 1);
+        // applying and emitting the remove should happen
+        // atomically, as updateNext/PreviousSibling might
+        // emit an update with the wrong index otherwise 
         this._tiles.splice(tileIdx, 1);
-        prevTile && prevTile.updateNextSibling(nextTile);
-        nextTile && nextTile.updatePreviousSibling(prevTile);
         tile.dispose();
         this.emitRemove(tileIdx, tile);
+        prevTile?.updateNextSibling(nextTile);
+        nextTile?.updatePreviousSibling(prevTile);
     }
 
     // would also be called when unloading a part of the timeline
@@ -301,5 +304,29 @@ export function tests() {
             entries.insert(1, {n: 7});
             assert(receivedAdd);
         },
+        "emit update with correct index in updatePreviousSibling during remove": assert => {
+            class UpdateOnSiblingTile extends TestTile {
+                updatePreviousSibling() {
+                    this.update?.(this, "previous");
+                }
+            }
+            const entries = new ObservableArray([{n: 5}, {n: 10}, {n: 15}]);
+            const tiles = new TilesCollection(entries, entry => new UpdateOnSiblingTile(entry));
+            const events = [];
+            tiles.subscribe({
+                onUpdate(idx, tile) {
+                    assert.equal(idx, 1);
+                    assert.equal(tile.entry.n, 15);
+                    events.push("update");
+                },
+                onRemove(idx, tile) {
+                    assert.equal(idx, 1);
+                    assert.equal(tile.entry.n, 10);
+                    events.push("remove");
+                }
+            });
+            entries.remove(1);
+            assert.deepEqual(events, ["remove", "update"]);
+        }
     }
 }

src/observable/list/ObservableArray.js

@@ -27,6 +27,11 @@ export class ObservableArray extends BaseObservableList {
         this.emitAdd(this._items.length - 1, item);
     }
 
+    remove(idx) {
+        const [item] = this._items.splice(idx, 1);
+        this.emitRemove(idx, item);
+    }
+
     insertMany(idx, items) {
         for(let item of items) {
             this.insert(idx, item);