In light of this feedback, we are landing a metric to understand how often subframe unload handlers run without main frame navigation. We'll use the data to adjust our plans accordingly.
Since it takes time to get & analyze the data, *we are pushing the proposed behavior change to M115*. *Adjustments based on feedback received thus far* We'll offer a "deprecation flag" (a flag to temporarily keep unload working as it currently does) which should be more convenient than the Extension suggestion (e.g. unmanaged devices where group policy can't be deployed). We are also removing the following aspect "This policy will also be enabled by default if Chrome detects that it is in an enterprise / education environment, as hinted by the presence of one or more existing group policies." from the proposal since we got feedback that it's better to keep group policy free of any automated logic. I've reached out to a few more communities and have only heard supportive comments so far (touching wood). Please continue to help us get the word out so that we can further minimize the risk of oversights. Thanks! On Thursday, March 30, 2023 at 3:23:16 PM UTC+9 Fergal Daly wrote: > [+sm...@mozilla.com] > > I'm relaying a piece of feedback from Mozilla in this github issue > <https://github.com/mozilla/standards-positions/issues/691#issuecomment-1484997320> > . > > It's possible that pages are depending on `unload` handlers in subframes > for functionality even without any main frame navigation. E.g a page > creates a subframe with an unload handler, when the subframe is destroyed > or navigates to somewhere else, that unload handler does something > interesting, e.g. notifies the outer frame that this has happened. > > This is definitely possible. It's also pretty easy to switch to pagehide > for this case but we should try to understand how common this is before > breaking it. It should be possible to measure how often subframe unloads > fire when the mainframe is not navigating. This will give us an upper bound > on the size of the problem, > > F > > > On Fri, 24 Mar 2023 at 10:16, Kenji Baheux <kenjibah...@chromium.org> > wrote: > >> Tl;dr: the presence of unload event listeners is a primary blocker for >> back/forward cache on Chromium based browsers and for Firefox on desktop >> platforms. On the other hand, for mobile platforms, almost all browsers >> prioritize the bfcache by not firing unload events in most cases. To >> improve the situation, we’ve been working with lots of partners and >> successfully reduced the use of unload event listeners over the last few >> years. To further accelerate this migration, we propose to have Chrome for >> desktop gradually skip unload events. If this call for feedback doesn’t >> unearth critical showstoppers and if the proposal makes it through the >> blink process, the behavior change could be starting from M114 at the >> earliest (note: beforeunload will remain unchanged). We’d like feedback >> on this plan, in particular use cases that don’t yet have a viable >> alternative. >> >> >> >> Hi, >> >> >> This is a call for feedback about a tentative plan regarding unload >> events. Our goal is to identify use cases for which there isn’t any good >> alternative to unload events, and would therefore prevent this plan from >> moving forward. >> >> >> The unload event is extremely unreliable. It is ignored in most cases by >> all mobile browsers with the exception of Firefox on Android. Furthermore, >> in Safari, the unload event is ignored on both desktop & mobile >> platforms. In addition to being unreliable, the presence of unload event >> listeners on a page is a major back/forward cache blocker on desktop for >> Chromium browsers and Firefox. Based on Chrome stats, we believe that >> unload event listeners reduce bfcache’s ability to deliver instant >> back/forward navigation by ~18 percentage points (hit-rate). >> >> >> Over the course of 2021~2022, we ran a large collaborative effort to >> reduce the usage of unload event listeners, in particular across popular >> third parties. We’ve seen great progress with many sites and third parties >> having already completed their migration. >> >> >> Given how unreliable unload events are, the potential user experience >> upsides, and the great progress achieved by the ecosystem on switching away >> from unload, we’d like to help accelerate the migration by gradually >> skipping unload events on Chrome for desktop. >> >> >> 👉 Please note that beforeunload will remain unchanged as this event >> doesn’t have reliability issues and doesn’t block BFCache. 👈 >> >> >> We are interested in hearing your feedback about this plan. In >> particular, please let us know if you are aware of unload event listener >> use cases that lack a viable alternative. Your feedback will inform the >> proposal (e.g. behavior and timeline). >> >> >> If this call for feedback doesn’t unearth any critical showstoppers, and >> if the proposal makes it through the blink process, we’d start the plan >> from M114 at the earliest by having a small likelihood of ignoring unload >> events while providing access to fine-tuning control (e.g. >> Permission-Policy: >> unload API <https://chromestatus.com/feature/5760325231050752>) and >> Enterprise/Edu carve outs. From there, we’ll continue to monitor the >> community’s feedback and gradually increase the likelihood over time. We >> are hoping to make significant progress by the end of this year, and hope >> to reach a satisfying state sometime in 2024. >> >> >> See the sections below for more context, our guidance for a post-unload >> web, an API to exert control over unload event listeners, and our >> approach to ease-in enterprise/edu products into this change. >> >> >> >> Background about bfcache >> >> Back/forward cache <https://web.dev/bfcache/> is a browser optimization >> that enables instant back and forward navigation. It’s an in-memory cache >> that stores a complete snapshot of a page (including the JavaScript heap) >> as the user is navigating away. With the entire page in memory, the browser >> can quickly and easily restore it >> <https://www.youtube.com/watch?v=cuPsdRckkF0> if the user decides to >> return. >> >> >> The multiple behaviors of bfcache with unload events >> >> Unfortunately, not all pages can be stored in bfcache. For instance, >> using certain APIs prevent pages from entering the bfcache. In particular, >> the presence of unload listeners on a page is the most common bfcache >> blocker. >> >> >> The use of unload listeners is highly discouraged because it’s a >> fundamentally unreliable event: >> >> - >> >> On desktop, Chrome and Firefox are currently firing unload events at >> the cost of the user experience, while Safari will attempt to cache some >> pages with an unload event listener (skipping the event in doing so). >> - >> >> On mobile, Chrome and Safari will attempt to cache pages with an >> unload event listener. On the other hand, Firefox treats pages that >> use unload event listeners as ineligible for the bfcache, except on >> iOS, which requires all browsers to use the WebKit rendering engine (i.e. >> all browsers inherently behave like Safari on this platform). >> >> >> Alternatives to unload event listener >> >> The recommended alternatives to unload event listeners are to: >> >> - >> >> Use the pagehide event listener >> >> <https://web.dev/bfcache/#only-add-beforeunload-listeners-conditionally:~:text=Instead%20of%20using%20the%20unload%20event%2C%20use%20the%20pagehide%20event.%20The%20pagehide%20event%20fires%20in%20all%20cases%20where%20the%20unload%20event%20currently%20fires%2C%20and%20it%20also%20fires%20when%20a%20page%20is%20put%20in%20the%20bfcache.> >> >> (note: despite the name, this serves a different purpose than the page >> visibility API). >> - >> >> For the cases where user interaction would be useful, conditionally >> use the beforeunload event listener >> <https://web.dev/bfcache/#only-add-beforeunload-listeners-conditionally> >> . >> - >> >> Use sendBeacon >> <https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon> >> or fetch keepalive >> >> <https://developer.mozilla.org/en-US/docs/Web/API/fetch#:~:text=keepalive,Navigator.sendBeacon()%20API.> >> >> to send analytics data. >> >> >> In addition, you may be interested in the origin trial >> <https://developer.chrome.com/origintrials/#/view_trial/1581889369113886721> >> for the Pending Beacon API >> <https://chromestatus.com/feature/5690553554436096>. This >> bfcache-friendly API allows sending a bundle of data to a backend server, >> ideally at the ‘end’ of a user’s visit to a page. From our observations, we >> believe this is the most common use case for unload event listeners. >> Compared to the methods highlighted above, this API has better ergonomics. >> >> >> Test driving a web free of unload event listeners! >> >> To understand how the plan might play out, please consider joining the >> origin >> trial >> <https://developer.chrome.com/origintrials/#/view_trial/1012184016251518977> >> for the Permissions-Policy: unload API >> <https://chromestatus.com/feature/5760325231050752>. This API allows any >> site to: >> >> - >> >> Exert control over unload event listeners (e.g. completely disallow >> them, or selectively allow them for specific origins). >> - >> >> Report the use of unload event listeners to an endpoint for >> assessment purposes. >> >> >> Chrome for Enterprise & Education >> >> We also acknowledge that providers of enterprise & education solutions >> may not always have the flexibility to quickly update existing deployments. >> To minimize concerns, we’ll offer a group policy to keep the current >> behavior for unload events. This policy will also be enabled by default >> if Chrome detects that it is in an enterprise / education environment, as >> hinted by the presence of one or more existing group policies. For >> unmanaged Enterprise/Edu environments, a simple Chrome extension could >> inject the relevant calls to the Permission-Policy:unload API for >> temporarily opting-out the relevant origin(s). >> > -- You received this message because you are subscribed to the Google Groups "blink-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to blink-dev+unsubscr...@chromium.org. To view this discussion on the web visit https://groups.google.com/a/chromium.org/d/msgid/blink-dev/dc362b13-2ca2-40bd-a3ac-e2dae29e3a61n%40chromium.org.
