fix(domains): restore fail-closed focus check in browser_type

Adopting isFocusProbe inverted the safety default: the guard was ANDed into
the condition gating the check, so a probe result the guard rejected skipped
the check and fell through to Input.insertText — text typed into nothing,
silently, which is the exact failure the block exists to prevent. The guard
now feeds a value that defaults to { ok: false }, so a rejected shape
produces the error instead of bypassing it.

The other four guard adoptions (isFillResult, isFocusResult, isSelectResult,
isPageResult) were already fail-closed: each feeds a value whose rejection
lands on a pre-existing error branch.

Also assert x/y in the box centre test — the fields snapshot's off-viewport
filter reads — and wrap the zero-size message to the surrounding width.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

3 files changed, 8 insertions, 7 deletions

diff --git a/src/domains/box.ts b/src/domains/box.ts
index 7b12e82..e013f70 100644
--- a/src/domains/box.ts
+++ b/src/domains/box.ts
@@ -41,7 +41,10 @@ export const boxOf = async (
     return err({ kind: "invalid_state", message: "DOM.getBoxModel returned an incomplete content quad" });
   }
   if (model.width <= 0 || model.height <= 0) {
-    return err({ kind: "invalid_state", message: "element has a zero-size box and cannot be clicked" });
+    return err({
+      kind: "invalid_state",
+      message: "element has a zero-size box and cannot be clicked",
+    });
   }
   return ok(centreOf(model));
 };
diff --git a/src/domains/keyboard.ts b/src/domains/keyboard.ts
index 21681c5..6722ffb 100644
--- a/src/domains/keyboard.ts
+++ b/src/domains/keyboard.ts
@@ -27,10 +27,6 @@ export const typeTool = defineBrowserTool({
   ],
   parameters: TypeArgs,
   async handler(args, { client }): Promise<Result<ToolOk, ToolErr>> {
-    // Guard against silent text loss: Input.insertText succeeds regardless of
-    // whether an editable element is focused, so without this check typed text
-    // can vanish into the void with no error. Fail loudly with an actionable
-    // message instead. IIFE starts with "(" so evaluateJs returns it directly.
     const focused = await client.evaluateJs(safeJs`
       (() => {
         const el = document.activeElement;
@@ -39,8 +35,8 @@ export const typeTool = defineBrowserTool({
         return { ok, tag: el ? el.tagName : null };
       })()
     `);
-    if (focused.success && isFocusProbe(focused.data)) {
-      const f = focused.data;
+    if (focused.success) {
+      const f: FocusProbe = isFocusProbe(focused.data) ? focused.data : { ok: false, tag: null };
       if (!f.ok) {
         return err({
           kind: "invalid_state",
diff --git a/test/domains/box-test.ts b/test/domains/box-test.ts
index cb930a0..248ae38 100644
--- a/test/domains/box-test.ts
+++ b/test/domains/box-test.ts
@@ -5,6 +5,8 @@ import { centreOf } from "../../src/domains/box";
 describe("box centre", () => {
   test("computes the centre from a content quad", () => {
     const b = centreOf({ content: [10, 20, 110, 20, 110, 70, 10, 70], width: 100, height: 50 });
+    assert.equal(b.x, 10);
+    assert.equal(b.y, 20);
     assert.equal(b.cx, 60);
     assert.equal(b.cy, 45);
     assert.equal(b.width, 100);