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webapi: invoke MutationObserver callbacks with the observer as this

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Fixes WPT /dom/nodes/MutationObserver-callback-arguments.html: per the
DOM spec ("invoke callback with a list of MutationRecord objects and mo,
and mo as callback this value"), the mutation callback's this value must
be the MutationObserver itself. We invoked it with the default receiver
(undefined -> globalThis), so `this === mo` failed.

deliverRecords now uses tryCallWithThis with the observer as receiver.

Coverage: /dom/nodes/MutationObserver-callback-arguments.html 0/1 -> 1/1
(fully green).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Francis Bouvier
2026-07-11 13:52:09 +02:00
committed by Karl Seguin
parent a603721f5f
commit 575ca0000c
1 changed files with 2 additions and 1 deletions
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3
src/browser/webapi/MutationObserver.zig
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@@ -350,7 +350,8 @@ pub fn deliverRecords(self: *MutationObserver, frame: *Frame) !void {
defer ls.deinit();
var caught: js.TryCatch.Caught = undefined;
ls.toLocal(self._callback).tryCall(void, .{ records, self }, &caught) catch |err| {
// Per spec, the callback is invoked with the observer as its this value.
ls.toLocal(self._callback).tryCallWithThis(void, self, .{ records, self }, &caught) catch |err| {
log.err(.frame, "MutObserver.deliverRecords", .{ .err = err, .caught = caught });
return err;
};