webapi: prevent click() from triggering click()

/html/editing/activation/click_checkbox.html is a DCHECK crash (debug only) and
just a failure in ReleaseFast.

click() handler shouldn't trigger its own click(). Click is singled out for this
behavior, which is 'do nothing':
https://html.spec.whatwg.org/multipage/interaction.html#click-in-progress-flag

Used the new Element flags. There's a simple way to implement this in
EventManager, but the element flag is cheaper and it's currently not used. If
someone ever needs that bit, they can implement the stack-based solution.

src/browser/tests/element/html/input_click.html

@@ -281,3 +281,30 @@
     testing.expectEqual(false, r3.checked);
   }
 </script>
+
+<script id="nested_click_is_noop">
+  {
+    // click() while a click() on the same element is in progress is a no-op
+    // (the "click in progress" flag); a re-entrant handler must not recurse.
+    const cb = document.createElement('input');
+    cb.type = 'checkbox';
+    document.body.appendChild(cb);
+
+    let clicks = 0;
+    cb.addEventListener('click', () => {
+      clicks++;
+      cb.click();
+    });
+
+    cb.click();
+    testing.expectEqual(1, clicks, 'nested click() should not dispatch');
+    testing.expectEqual(true, cb.checked);
+
+    // The flag is cleared afterwards, so a fresh click() works again.
+    cb.click();
+    testing.expectEqual(2, clicks);
+    testing.expectEqual(false, cb.checked);
+
+    document.body.removeChild(cb);
+  }
+</script>

src/browser/webapi/Element.zig

@@ -139,7 +139,14 @@ pub const Namespace = enum(u8) {
 pub const Flags = packed struct(u8) {
     shadow_host: bool = false,
     customized_builtin: bool = false,
-    _unused: u6 = 0,
+
+    // Prevents nested clicks (which have a specific spec-compliant behavior
+    // compared to other events). If this bit can be more useful for something
+    // else, a stack in EventManager (for click-specifically) is an alterantive
+    // approach
+    click_in_progress: bool = false,
+
+    _unused: u5 = 0,
 };
 
 _type: Type,

src/browser/webapi/element/Html.zig

@@ -402,6 +402,13 @@ pub fn click(self: *HtmlElement, frame: *Frame) !void {
         else => {},
     }
 
+    const flags = &self.asElement()._flags;
+    if (flags.click_in_progress) {
+        return;
+    }
+    flags.click_in_progress = true;
+    defer flags.click_in_progress = false;
+
     const event = (try @import("../event/MouseEvent.zig").init("click", .{
         .bubbles = true,
         .cancelable = true,