fix(input): honor mousedown preventDefault and correct mouse-focus rules

Addresses review on the mousedown default action.

Blur when a mousedown lands outside any focusable element: focusForMouseDown
now blurs the document's active element when the ancestor walk finds nothing
mouse-focusable, so a plain click moves focus to the body. The MCP fixture had
this backwards and asserted the old behavior; it now records
document.activeElement === document.body.

Treat an unparsable tabindex as if the attribute were absent (HTML 6.6.3)
rather than as a terminal "not focusable" answer, so <button tabindex="abc">
keeps its native focusability. This mirrors HtmlElement.getTabIndex's
parse-failure fallthrough for the same attribute instead of introducing a
second parser with different semantics.

Check the tabindex attribute before the HTML-only guard. An explicit,
parseable tabindex is focusable on any element, and SVG links are focusable by
the same href they activate on. This was only a gap until the blur above made
it a regression: clicking such an element used to leave focus alone and would
now have dropped it to the body, where Chrome focuses the element.

Honor preventDefault() on mousedown. The default action ran unconditionally,
so the toolbar idiom -- preventDefault() on mousedown to keep focus in a
focused input -- did not work, and the new blur made that load-bearing. The
mouse dispatch helpers in actions.zig, user_input.zig and WebDriver.zig now
report whether the event was cancelled, and each call site gates
focusForMouseDown on it. Non-mousedown call sites discard the result.

Editing-host focus behavior is deliberately unchanged.

Fold mouseFocusTabIndex into isMouseFocusable; its ?i32 return was never read.
Drop the duplicate Runtime.zig test that drove the same actions.click path as
the tools.zig fixture, and expand the remaining one, renamed to reflect what it
now covers. Two of its assertions sample document.activeElement in a mouseup
listener rather than after the click, because click activation behavior focuses
those elements unconditionally and would mask what mousedown decided. Harden
the disabled-control test to record the full five-event sequence instead of
mousedown alone. Every assertion was checked to fail against the pre-fix code.

zig build test 1340/1340, zig build test -Dwpt_extensions 1340/1340,
zig fmt --check clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/script/Runtime.zig

@@ -1441,7 +1441,13 @@ test "agent script runtime: tool errors throw and stop execution" {
     );
 }
 
-test "agent script runtime: selector click preserves pointer mouse semantics" {
+// Complements the fixture-driven MCP test in tools.zig: here the nodes are
+// created at runtime. Covers the ancestor walk, blur-to-body on a non-focusable
+// node, tabindex=-1 still mouse-focusable, an unparsable tabindex on a plain div
+// not focusable but on a button falling through to native focusability,
+// preventDefault() on mousedown preserving focus, editing hosts winning over
+// their contents, and an explicit tabindex on a non-HTML (SVG) element.
+test "agent script runtime: mousedown focus follows mouse-focusability rules" {
     defer testing.test_session.closeAllPages();
 
     var registry = CDPNode.Registry.init(testing.allocator);
@@ -1453,41 +1459,44 @@ test "agent script runtime: selector click preserves pointer mouse semantics" {
     try runTestScript(runtime,
         \\const page = new Page();
         \\await page.goto("http://localhost:9582/src/browser/tests/mcp_actions.html");
-        \\page.click("#btn");
-        \\const full = page.evaluate("JSON.stringify(window.seq)");
-        \\if (full !== '["pointerdown:0:1:mouse:true","mousedown:0:1::true","pointerup:0:0:mouse:true","mouseup:0:0::true","click:0:0:mouse:true"]') throw new Error("wrong full click sequence: " + full);
-        \\page.click("#btnPreventDefault");
-        \\const suppressed = page.evaluate("JSON.stringify(window.seqPrevented)");
-        \\if (suppressed !== '["pointerdown","pointerup","click"]') throw new Error("wrong suppressed sequence: " + suppressed);
-        \\page.click("#btnDisabled");
-        \\if (page.evaluate("String(window.disabledMousedowned)") !== "false") throw new Error("disabled button received mousedown");
-        \\page.click("#focusTarget");
-        \\if (page.evaluate("document.activeElement.id") !== "focusTarget") throw new Error("mousedown focus default action was lost");
-    );
-}
-
-// Different workflow from the fixture above: the tabindex node is created at
-// runtime, the click lands on a child (ancestor walk), a following click on a
-// non-focusable node must not steal focus, and tabindex=-1 is still mouse-focusable.
-test "agent script runtime: dynamic tabindex child click focuses ancestor" {
-    defer testing.test_session.closeAllPages();
-
-    var registry = CDPNode.Registry.init(testing.allocator);
-    defer registry.deinit();
-
-    const runtime = try Runtime.init(testing.allocator, testing.test_app, testing.test_session, &registry);
-    defer runtime.deinit();
-
-    try runTestScript(runtime,
-        \\const page = new Page();
-        \\await page.goto("http://localhost:9582/src/browser/tests/mcp_actions.html");
-        \\page.evaluate("const p=document.createElement('div');p.id='dynFocus';p.setAttribute('tabindex','0');const s=document.createElement('span');s.id='dynChild';s.textContent='x';p.appendChild(s);document.body.appendChild(p);const n=document.createElement('div');n.id='dynNeg';n.setAttribute('tabindex','-1');n.textContent='neg';document.body.appendChild(n)");
+        \\page.evaluate("const p=document.createElement('div');p.id='dynFocus';p.setAttribute('tabindex','0');const s=document.createElement('span');s.id='dynChild';s.textContent='x';p.appendChild(s);document.body.appendChild(p);const n=document.createElement('div');n.id='dynNeg';n.setAttribute('tabindex','-1');n.textContent='neg';document.body.appendChild(n);const b=document.createElement('div');b.id='dynBad';b.setAttribute('tabindex','abc');b.textContent='bad';document.body.appendChild(b);const btn=document.createElement('button');btn.id='dynBadBtn';btn.setAttribute('tabindex','abc');btn.textContent='badbtn';btn.addEventListener('mouseup',function(){window.badBtnFocusAtMouseup=document.activeElement.id;});document.body.appendChild(btn)");
         \\page.click("#dynChild");
         \\if (page.evaluate("document.activeElement.id") !== "dynFocus") throw new Error("child click did not focus tabindex ancestor: " + page.evaluate("document.activeElement && document.activeElement.id"));
         \\page.click("#plain");
-        \\if (page.evaluate("document.activeElement.id") !== "dynFocus") throw new Error("plain click stole focus: " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\if (page.evaluate("String(document.activeElement === document.body)") !== "true") throw new Error("plain click did not blur to body: " + page.evaluate("document.activeElement && document.activeElement.id"));
         \\page.click("#dynNeg");
         \\if (page.evaluate("document.activeElement.id") !== "dynNeg") throw new Error("tabindex=-1 was not mouse-focusable: " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\page.click("#dynBad");
+        \\if (page.evaluate("String(document.activeElement === document.body)") !== "true") throw new Error("unparsable tabindex was mouse-focusable: " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\page.click("#dynBadBtn");
+        \\// Sampled at mouseup, not after the click: a button's click activation
+        \\// behavior focuses it unconditionally, which would mask what mousedown did.
+        \\if (page.evaluate("window.badBtnFocusAtMouseup") !== "dynBadBtn") throw new Error("unparsable tabindex on a native button lost native mousedown focusability: " + page.evaluate("String(window.badBtnFocusAtMouseup)"));
+        \\// Toolbar idiom: preventDefault() on mousedown preserves existing focus
+        \\page.evaluate("const t=document.createElement('div');t.id='toolbarBtn';t.addEventListener('mousedown',function(e){e.preventDefault();});document.body.appendChild(t)");
+        \\page.click("#inp");
+        \\if (page.evaluate("document.activeElement.id") !== "inp") throw new Error("setup failed: activeElement not inp");
+        \\page.click("#toolbarBtn");
+        \\if (page.evaluate("document.activeElement.id") !== "inp") throw new Error("preventDefault on mousedown did not protect focus (toolbar idiom): " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\// Editing hosts: the click focuses the host, not what is inside it.
+        \\// Verified against Chrome 152 with trusted CDP input: a plain span, a
+        \\// button, a [tabindex] div and an <a href> inside a contenteditable all
+        \\// leave focus on the host, and a nested host resolves to the OUTER one.
+        \\page.evaluate("const h=document.createElement('div');h.id='dynHost';h.setAttribute('contenteditable','true');const hs=document.createElement('span');hs.id='dynHostSpan';hs.textContent='hs';h.appendChild(hs);document.body.appendChild(h)");
+        \\page.click("#dynHostSpan");
+        \\if (page.evaluate("document.activeElement.id") !== "dynHost") throw new Error("span inside contenteditable did not focus host: " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\// Nested contenteditable: the OUTER host wins, not the inner one.
+        \\page.evaluate("const out=document.createElement('div');out.id='dynOuter';out.setAttribute('contenteditable','true');const inn=document.createElement('div');inn.id='dynInner';inn.setAttribute('contenteditable','true');const is=document.createElement('span');is.id='dynInnerSpan';is.textContent='is';inn.appendChild(is);out.appendChild(inn);document.body.appendChild(out)");
+        \\page.click("#dynInnerSpan");
+        \\if (page.evaluate("document.activeElement.id") !== "dynOuter") throw new Error("nested contenteditable did not focus the outer host: " + page.evaluate("document.activeElement && document.activeElement.id"));
+        \\// An explicit tabindex is focusable on a non-HTML element too: the
+        \\// HTML-only guard must not shadow it, or the new blur fallback would
+        \\// drop focus to body on a click Chrome focuses.
+        \\page.evaluate("const svg=document.createElementNS('http://www.w3.org/2000/svg','svg');const r=document.createElementNS('http://www.w3.org/2000/svg','rect');r.id='dynSvgRect';r.setAttribute('tabindex','0');r.setAttribute('width','100');r.setAttribute('height','40');svg.appendChild(r);document.body.appendChild(svg)");
+        \\page.click("#inp");
+        \\if (page.evaluate("document.activeElement.id") !== "inp") throw new Error("setup failed: activeElement not inp");
+        \\page.click("#dynSvgRect");
+        \\if (page.evaluate("document.activeElement.id") !== "dynSvgRect") throw new Error("svg [tabindex] click did not focus the svg element: " + page.evaluate("document.activeElement && document.activeElement.id"));
     );
 }