Don't crash if Firefox hands us negative bookmark indices [#417]

Firefox will sometimes report that bookmarks are at negative indices in
their parent folder, because of internal inconsistencies in the user's
bookmarks DB. Work around this by clamping the index in `setPosition()`
to positive values.  We'll probably position the bookmark incorrectly,
but we at least won't crash.

src/model/tree.test.ts

@@ -320,5 +320,31 @@ describe("model/tree", () => {
         );
       },
     );
+
+    it("deals with gaps in the indexing", () => {
+      setPosition(nodes.d, {
+        parent: parents.root,
+        index: 10,
+      });
+      expect(parents.root.children).to.deep.equal(
+        ["a", "b", "c", "e", "f", "d"].map(i => nodes[i]),
+      );
+      expect(nodes.d.position?.parent).to.equal(parents.root);
+      expect(nodes.d.position?.index).to.equal(5);
+    });
+
+    it("deals with negative indexes", () => {
+      setPosition(nodes.d, {
+        parent: parents.root,
+        index: -2,
+      });
+      expect(parents.root.children).to.deep.equal(
+        ["d", "a", "b", "c", "e", "f"].map(i => nodes[i]),
+      );
+      expect(nodes.d.position?.parent).to.equal(parents.root);
+      expect(nodes.d.position?.index).to.equal(0);
+      expect(nodes.a.position?.index).to.equal(1);
+      expect(nodes.b.position?.index).to.equal(2);
+    });
   });
 });

src/model/tree.ts

@@ -103,7 +103,9 @@ export function setPosition<
 
   if (newPosition) {
     const newChildren = newPosition.parent.children;
-    if (newPosition.index > newChildren.length) {
+    if (newPosition.index < 0) {
+      newPosition.index = 0;
+    } else if (newPosition.index > newChildren.length) {
       newPosition.index = newChildren.length;
     }