1797263 - alert prevents rendering of page

Status: RESOLVED WONTFIX

(Core :: DOM: Core & HTML, defect)

Description

[David Balažic]

Attachment: reddit page (without CSS) with the added JavaScript alert

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:106.0) Gecko/20100101 Firefox/106.0

Steps to reproduce:

as described on https://www.reddit.com/r/firefox/comments/yczdap/alert_in_documentready_creates_page_rendering/

(copied description)

I noticed recently on some web pages that an alert will make the "background" (the page) not render correctly.

What happens is that most of the page is loaded/displayed and the alert is shown, but large parts of the page are missing. If I ctrl+tab to another tab and back, the missing content appears. The same happens if I slightly change the window size.

Regression in FF 106 ? I don't remember seeing this in older versions, but can't be sure.

PS: I saved this webpage (just the HTML), added this at the end of the body and the same thing happens:

<script>
document.addEventListener("DOMContentLoaded", function(event) {
alert("tu sem");
});
</script>

The problem happens in 2 tries out of 3.

(example HTML file attached)

Comment 1

[David Balažic]

I have Firefox version 106.0.1

The problem happens with a fresh profile too.

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'Core::DOM: Core & HTML' component, and is moving the bug to that component. Please correct in case you think the bot is wrong.

Comment 3

[Sean Feng [:sefeng]]

Using ./mach mozregression --bad 2022-10-25 --good 105 command, I can see the behaviour changes since bug 1788125. Thought the behaviour got changed to the alert blocks the rendering of the entire document (not partially as to what is being described in the description), probably the behaviour got changed again later with some further tweaks.

Olli, mind take a look?

Comment 4

[Olli Pettay [:smaug][[e-mail]]]

That is the expected behavior from bug 1788125. We shouldn't really paint since that means running animation frame callbacks during alert().

Comment 5

[Olli Pettay [:smaug][[e-mail]]]

And tab switch is causing a forced paint without triggering rAF callbacks. We could possibly do some of that, but it would also regress bug 1788125.

Comment 6

[Sean Feng [:sefeng]]

Okay, since this is expected, I am closing this as WONTFIX

Comment 7

[David Balažic]

Additional information: Sometimes the previous web page is displayed instead of the current one

Details:

• open a web page
• submit (POST) a form
• the new page displays an alert
• the web page shown is the old one, not the new one