Bugfix: Do not unhide a suspended tree without finishing the suspended update #18411
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When we commit a fallback, we cannot unhide the content without including the level that originally suspended. That's because the work at level outside the boundary (i.e. everything that wasn't hidden during that render) already committed.
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I think this was just poor factoring on my part in facebook#18411. Honestly it doesn't make much sense to me, but my best guess is that I must have though that when `baseTime > currentChildExpirationTime`, the function would fall through to the `currentChildExpirationTime < renderExpirationTime` branch below. Really I think just made an oopsie. Regardless, this logic is galaxy brainéd. A goal of the Lanes refactor I'm working on is to make these types of checks -- is there remaining work in this tree? -- a lot easier to think about. Hopefully.
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I think this was just poor factoring on my part in facebook#18411. Honestly it doesn't make much sense to me, but my best guess is that I must have thought that when `baseTime > currentChildExpirationTime`, the function would fall through to the `currentChildExpirationTime < renderExpirationTime` branch below. Really I think just made an oopsie. Regardless, this logic is galaxy brainéd. A goal of the Lanes refactor I'm working on is to make these types of checks -- is there remaining work in this tree? -- a lot easier to think about. Hopefully.
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* Failing test for #18657 * Remove incorrect priority check I think this was just poor factoring on my part in #18411. Honestly it doesn't make much sense to me, but my best guess is that I must have thought that when `baseTime > currentChildExpirationTime`, the function would fall through to the `currentChildExpirationTime < renderExpirationTime` branch below. Really I think just made an oopsie. Regardless, this logic is galaxy brainéd. A goal of the Lanes refactor I'm working on is to make these types of checks -- is there remaining work in this tree? -- a lot easier to think about. Hopefully.
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Apr 25, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manfiested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
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Apr 25, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
acdlite
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Apr 25, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
acdlite
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Apr 29, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
acdlite
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May 1, 2020
When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in facebook#18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in facebook#18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root.
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May 1, 2020
* Unhide Suspense trees without entanglement When a Suspense boundary is in its fallback state, you cannot switch back to the main content without also finishing any updates inside the tree that might have been skipped. That would be a form of tearing. Before we fixed this in #18411, the way this bug manifested was that a boundary was suspended by an update that originated from a child component (as opposed to props from a parent). While the fallback was showing, it received another update, this time at high priority. React would render the high priority update without also including the original update. That would cause the fallback to switch back to the main content, since the update that caused the tree to suspend was no longer part of the render. But then, React would immediately try to render the original update, which would again suspend and show the fallback, leading to a momentary flicker in the UI. The approach added in #18411 is, when receiving a high priority update to a Suspense tree that's in its fallback state is to bail out, keep showing the fallback and finish the update in the rest of the tree. After that commits, render again at the original priority. Because low priority expiration times are inclusive of higher priority expiration times, this ensures that all the updates are committed together. The new approach in this commit is to turn `renderExpirationTime` into a context-like value that lives on the stack. Then, when unhiding the Suspense boundary, we can push a new `renderExpirationTime` that is inclusive of both the high pri update and the original update that suspended. Then the boundary can be unblocked in a single render pass. An advantage of the old approach is that by deferring the work of unhiding, there's less work to do in the high priority update. The key advantage of the new approach is that it solves the consistency problem without having to entangle the entire root. * Create internal LegacyHidden type This only exists so we can clean up the internal implementation of `<div hidden={isHidden} />`, which is not a stable feature. The goal is to move everything to the new Offscreen type instead. However, Offscreen has different semantics, so before we can remove the legacy API, we have to migrate our internal usage at Facebook. So we'll need to maintain both temporarily. In this initial commit, I've only added the type. It's not used anywhere. The next step is to use it to implement `hidden`. * Use LegacyHidden to implement old hidden API If a host component receives a `hidden` prop, we wrap its children in an Offscreen fiber. This is similar to what we do for Suspense children. The LegacyHidden type happens to share the same implementation as the new Offscreen type, for now, but using separate types allows us to fork the behavior later when we implement our planned changes to the Offscreen API. There are two subtle semantic changes here. One is that the children of the host component will have their visibility toggled using the same mechanism we use for Offscreen and Suspense: find the nearest host node children and give them a style of `display: none`. We didn't used to do this in the old API, because the `hidden` DOM attribute on the parent already hides them. So with this change, we're actually "overhiding" the children. I considered addressing this, but I figure I'll leave it as-is in case we want to expose the LegacyHidden component type temporarily to ease migration of Facebook's internal callers to the Offscreen type. The other subtle semantic change is that, because of the extra fiber that wraps around the children, this pattern will cause the children to lose state: ```js return isHidden ? <div hidden={true} /> : <div />; ``` The reason is that I didn't want to wrap every single host component in an extra fiber. So I only wrap them if a `hidden` prop exists. In the above example, that means the children are conditionally wrapped in an extra fiber, so they don't line up during reconciliation, so they get remounted every time `isHidden` changes. The fix is to rewrite to: ```js return <div hidden={isHidden} />; ``` I don't anticipate this will be a problem at Facebook, especially since we're only supposed to use `hidden` via a userspace wrapper component. (And since the bad pattern isn't very React-y, anyway.) Again, the eventual goal is to delete this completely and replace it with Offscreen.
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Jun 7, 2022
This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: facebook#18411.) The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes. The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing. The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way. The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates. The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update. The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition. To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in facebook#24663. Because this bugfix depends on a moderately risky refactor to the update queue (facebook#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork.
acdlite
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Jun 7, 2022
This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: facebook#18411.) The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes. The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing. The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way. The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates. The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update. The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition. To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in facebook#24663. Because this bugfix depends on a moderately risky refactor to the update queue (facebook#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork.
acdlite
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Jun 7, 2022
This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: facebook#18411.) The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes. The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing. The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way. The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates. The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update. The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition. To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in facebook#24663. Because this bugfix depends on a moderately risky refactor to the update queue (facebook#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork.
acdlite
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Jun 7, 2022
This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: facebook#18411.) The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes. The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing. The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way. The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates. The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update. The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition. To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in facebook#24663. Because this bugfix depends on a moderately risky refactor to the update queue (facebook#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork.
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* Add `isHidden` to OffscreenInstance We need to be able to read whether an offscreen tree is hidden from an imperative event. We can store this on its OffscreenInstance. We were already scheduling a commit effect whenever the visibility changes, in order to toggle the inner effects. So we can reuse that. * [FORKED] Bugfix: Revealing a hidden update This fixes a bug I discovered related to revealing a hidden Offscreen tree. When this happens, we include in that render all the updates that had previously been deferred — that is, all the updates that would have already committed if the tree weren't hidden. This is necessary to avoid tearing with the surrounding contents. (This was the "flickering" Suspense bug we found a few years ago: #18411.) The way we do this is by tracking the lanes of the updates that were deferred by a hidden tree. These are the "base" lanes. Then, in order to reveal the hidden tree, we process any update that matches one of those base lanes. The bug I discovered is that some of these base lanes may include updates that were not present at the time the tree was hidden. We cannot flush those updates earlier that the surrounding contents — that, too, could cause tearing. The crux of the problem is that we sometimes reuse the same lane for base updates and for non-base updates. So the lane alone isn't sufficient to distinguish between these cases. We must track this in some other way. The solution I landed upon was to add an extra OffscreenLane bit to any update that is made to a hidden tree. Then later when we reveal the tree, we'll know not to treat them as base updates. The extra OffscreenLane bit is removed as soon as that lane is committed by the root (markRootFinished) — at that point, it gets "upgraded" to a base update. The trickiest part of this algorithm is reliably detecting when an update is made to a hidden tree. What makes this challenging is when the update is received during a concurrent event, while a render is already in progress — it's possible the work-in-progress render is about to flip the visibility of the tree that's being updated, leading to a race condition. To avoid a race condition, we will wait to read the visibility of the tree until the current render has finished. In other words, this makes it an atomic operation. Most of this logic was already implemented in #24663. Because this bugfix depends on a moderately risky refactor to the update queue (#24663), it only works in the "new" reconciler fork. We will roll it out gradually to www before landing in the main fork. * Add previous commit to list of forked revisions
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This code was originally added in the old ExpirationTime implementation of Suspense. The idea is that if multiple updates suspend inside the same Suspense boundary, and both of them resolve, we should render both results in the same batch, to reduce jank. This was an incomplete idea, though. We later discovered a stronger requirement — once we show a fallback, we cannot fill in that fallback without completing _all_ the updates that were previously skipped over. Otherwise you get tearing. This was fixed by facebook#18411, then we discovered additional related flaws that were addressed in facebook#24685. See those PR descriptions for additional context. So I believe this older code is no longer necessary.
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This code was originally added in the old ExpirationTime implementation of Suspense. The idea is that if multiple updates suspend inside the same Suspense boundary, and both of them resolve, we should render both results in the same batch, to reduce jank. This was an incomplete idea, though. We later discovered a stronger requirement — once we show a fallback, we cannot fill in that fallback without completing _all_ the updates that were previously skipped over. Otherwise you get tearing. This was fixed by facebook#18411, then we discovered additional related flaws that were addressed in facebook#24685. See those PR descriptions for additional context. So I believe this older code is no longer necessary.
acdlite
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Jan 4, 2023
This code was originally added in the old ExpirationTime implementation of Suspense. The idea is that if multiple updates suspend inside the same Suspense boundary, and both of them resolve, we should render both results in the same batch, to reduce jank. This was an incomplete idea, though. We later discovered a stronger requirement — once we show a fallback, we cannot fill in that fallback without completing _all_ the updates that were previously skipped over. Otherwise you get tearing. This was fixed by facebook#18411, then we discovered additional related flaws that were addressed in facebook#24685. See those PR descriptions for additional context. So I believe this older code is no longer necessary.
acdlite
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Jan 4, 2023
This code was originally added in the old ExpirationTime implementation of Suspense. The idea is that if multiple updates suspend inside the same Suspense boundary, and both of them resolve, we should render both results in the same batch, to reduce jank. This was an incomplete idea, though. We later discovered a stronger requirement — once we show a fallback, we cannot fill in that fallback without completing _all_ the updates that were previously skipped over. Otherwise you get tearing. This was fixed by #18411, then we discovered additional related flaws that were addressed in #24685. See those PR descriptions for additional context. So I believe this older code is no longer necessary.
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This code was originally added in the old ExpirationTime implementation of Suspense. The idea is that if multiple updates suspend inside the same Suspense boundary, and both of them resolve, we should render both results in the same batch, to reduce jank. This was an incomplete idea, though. We later discovered a stronger requirement — once we show a fallback, we cannot fill in that fallback without completing _all_ the updates that were previously skipped over. Otherwise you get tearing. This was fixed by #18411, then we discovered additional related flaws that were addressed in #24685. See those PR descriptions for additional context. So I believe this older code is no longer necessary. DiffTrain build for [48274a4](48274a4) [View git log for this commit](https://github.com/facebook/react/commits/48274a43aa708f63a7580142a4c1c1a47f31c1ac)
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Fixes #18357
When we commit a fallback, we cannot unhide the content without including the level that originally suspended. That's because the work at level outside the boundary (i.e. everything that wasn't hidden during that render) already committed.