Bug 244559 – No-op textContent replacement could be short-circuited, like in Chrome

• Status: NEW
• Component: DOM

[Ahmad Saleem]

Hi Team,

While I was trying to triage old bug, I came across a test case, which is causing me difficult to post results in the browsers and selection is getting invalidating continuously and shuttering.

Link - https://persistent.info/webkit/test-cases/selection-deletion.html

Bug - https://bugs.webkit.org/show_bug.cgi?id=110682

<< STEPS TO REPRODUCE >>>

1) Open Link

2) Try selecting "DOM" text and see "Selection will flash.

BROWSER - Safari Technology Preview 152 and Safari 15.6.1 on macOS 12.5.1

3) Now try to open "Inspector" and select "DOM" and try to copy text within "DOM",  you will not be able to copy and selection will get lost.

<< ACTUAL RESULTS >>

Selection fails within browser view and in Web Inspector as well.

<< EXPECTED RESULT >>

Let me copy my text.. :-)

I though to create this bug in separate to make it easier to track and fix.

Thanks!

[Alexey Proskuryakov]

The reason why this is happening in Safari and in Firefox is that the page replaces div.textContent every 100ms.

The reason why this doesn't happen in Chrome is that this operation is short-circuited there, the Node isn't replaced when the text isn't changing.

This optimization is not allowed per DOM Living Standard, but maybe the spec and WebKit can change if Chrome is getting away with it.

Easy to observe with a test like this:

<div id=div>aaa</div>

e = document.getElementById("div").firstChild
document.getElementById("div").textContent = "aaa";
e === document.getElementById("div").firstChild
# true in Chrome, false in other browsers.

[Karl Dubost]

> The textContent setter steps are to, if the given value is null, act as if it was the empty string instead, and then do as described below, switching on the interface this implements:
>
> …
> 
> CharacterData
> Replace data with node this, offset 0, count this’s length, and data the given value.
in https://dom.spec.whatwg.org/#dom-node-textcontent



and replace data
in https://dom.spec.whatwg.org/#concept-cd-replace

For the chrome optimization
https://github.com/chromium/chromium/blob/924ec189cdfd33c8cee15d918f927afcb88d06db/third_party/blink/renderer/core/dom/node.cc#L2036-L2041

pointing to https://bugs.chromium.org/p/chromium/issues/detail?id=352836
as the source of the optimization.

[Alexey Proskuryakov]

My steps in comment 1 aren't true to the original test case, which modifies document.querySelector("#dom").textContent. So this still seems like a Chrome bug per the spec?

> Element
> String replace all with the given value within this.

[Karl Dubost]

yup, per spec.
But I wonder if it could become an interesting optimization for all. Or if it has unintended consequences.

[Anne van Kesteren]

Filed https://github.com/whatwg/dom/issues/1106 to discuss. Thanks for raising this Ahmad!

[Darin Adler]

One irony here is that in Safari’s early days we had a many "do less work because nothing changed" optimizations; part of "growing up" as a web browser engine was removing those because they harmed interoperability. I hope we do all agree to make this optimization a standard thing; given our shared WebKit roots I wonder when/why WebKit and Chromium diverged on this point.

[Radar WebKit Bug Importer]

<rdar://problem/99624320>

[Myles C. Maxfield]

Should we mark this bug as MOVED while https://github.com/whatwg/dom/issues/1106 is being discussed?

[Alexey Proskuryakov]

We usually don't do this. I don't think that it's beneficial to hide it in RESOLVED state when it's an observable compatibility issue.

[Ahmad Saleem]

I am not sure whether it is same bug or not but I came across another such case:

https://jsfiddle.net/2kEZ7/1/show

Try to copy the numbers / result from rotated cube / square:

getBoundingClientRect().width of div element is: xxxx <----- can't select this text.

It happens in Chrome Canary 108, Safari 16 while Firefox Nightly 107 does allow you to copy it while clicking on the line and double clicking but not when you are trying to double click on the value.

Just wanted to share. Thanks!