Bug 188499 - Setting a frame src after appending it to the DOM makes the frame's load event be fired twice
Summary: Setting a frame src after appending it to the DOM makes the frame's load even...
Status: NEW
Alias: None
Product: WebKit
Classification: Unclassified
Component: Frames (show other bugs)
Version: Safari 11
Hardware: Macintosh Unspecified
: P2 Normal
Assignee: Nobody
Keywords: InRadar
Depends on:
Reported: 2018-08-12 23:53 PDT by Timothée Pillard
Modified: 2019-09-18 02:15 PDT (History)
7 users (show)

See Also:

Test case (277 bytes, text/html)
2018-08-20 12:24 PDT, Ryosuke Niwa
no flags Details

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Description Timothée Pillard 2018-08-12 23:53:24 PDT

I am creating this bug in follow-up to https://twitter.com/tpillard/status/1028663435524009984

The following JavaScript code will log the "Loaded" message twice.

const root = document.body;
const frame = document.createElement('iframe');
const href = 'https://bugs.webkit.org';

frame.onload = (evt) => console.log('Loaded', evt);
frame.src = href;

Inverting the last two lines (setting the frame's src before appending it to the DOM) fixes the problem.

Tested and fixed with the following UA string:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_5) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.1.1 Safari/605.1.15

I've originally had some customer complaints on this behavior, one includes the following UA string:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/11.1 Safari/605.1.15

While debugging, I noticed similar issues have been reported by users on StackOverflow over the years:

I could not reproduce this behavior on other browsers I had at my disposal.

Thank you for any response,

Comment 1 Timothée Pillard 2018-08-13 00:05:52 PDT
Quick note:

Setting the `load` event handler on the iframe after appending it to the DOM fixes the problem as well.
Comment 2 Ryosuke Niwa 2018-08-20 12:24:29 PDT
Created attachment 347527 [details]
Test case
Comment 3 Ryosuke Niwa 2018-08-20 12:28:02 PDT
On interesting, When an iframe is inserted into a document, it loads about:blank which in turn fires a load event. What makes WebKit and Blink's behavior different from the spec is that we synchronously fire a load event in this case.

See https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element

When an iframe element is inserted into a document that has a browsing context, the user agent must create a new browsing context, set the element's nested browsing context to the newly-created browsing context, and then process the iframe attributes for the "first time".


Otherwise, if the element has no src attribute specified, and the user agent is processing the iframe's attributes for the "first time"
Queue a task to run the iframe load event steps.
The task source for this task is the DOM manipulation task source.
Comment 4 Ryosuke Niwa 2018-08-20 12:28:51 PDT
My guess is that making the event load fire async won't be a Web compatible change at this point but we can try.
Comment 5 Radar WebKit Bug Importer 2018-11-12 16:27:33 PST