bug(drag-drop): Layout thrashing (forced layout reflow) when dragging between dropzones with 400+ complexed drag refs
- Dominant language
- TypeScript
- Stars
- 25k
- Forks
- 6.8k
- Avg merge
- 1d 8h
- Merged PRs (30d)
- 91
Description
### Is this a regression?
- [ ] Yes, this behavior used to work in the previous version
### The previous version in which this bug was not present was
_No response_
### Description
### Problem
The layout thrashing problem occurs when the user drag the complex element between drop zones.


### There are 42 drop zones.

### There are 400+ drag refs.

###
The drag refs are quite a bit complex. We have several kind of drag Refs. For example, we have a text component which including Text Editor (Quill) inside.

However, we think the number of components and drag ref complexity of root element are not the main reason for layout thrashing.
In the investigation, there are two cases make layout thrashing when dragging between drop zones.
The first is in extendStyles in _toggleNativeDragInteractions in _startDragSequence, and then call **_cacheItemPositions** for **loop** each item in a drop zone which call **getMutableClientRect** which call **getBoundingClientRect**.



It extends styles from [toggleVisibility](https://github.com/angular/components/blob/main/src/cdk/drag-drop/drag-ref.ts#L825) (WRITE), insert placeholder, in body ` this._document.body.appendChild(parent.replaceChild(placeholder, element));` [Line no. 826](https://github.com/angular/components/blob/main/src/cdk/drag-drop/drag-ref.ts#L826) (WRITE) and getBoundingClientRect in [_cacheItemPositions](https://github.com/angular/components/blob/main/src/cdk/drag-drop/sorting/single-axis-sort-strategy.ts#L304) (READ).
Suggested Solution
1. _cacheItemsPositions, getBoundingClientRect (READ) first, appendChild, extend Styles (WRITE) later. Not sure is it possible.
(I read it from [avoid-layout-thrashing thrashing](https://web.dev/avoid-large-complex-layouts-and-layout-thrashing/#avoid-layout-))
2. Same code, but batching read write DOM i.e. [FastDOM](https://github.com/wilsonpage/fastdom)
3. The drag drop library mandatory insert placeholder at _startDragSequence and _endDragSequence which is not optimum. I think placeholder should be optional, and also optional extend styles.
That's all for the first layout thrashing.
The second is almost the same as the first but a little difference. This always happen when dragging item changing active container. This layout thrashing always makes Janky user experience.

It first starts from **_updateActiveDropContainer** function.
In the investigation, the [enter function](https://github.com/angular/components/blob/main/src/cdk/drag-drop/sorting/single-axis-sort-strategy.ts#L175) in SingleAxisSortStrategy class has insertBefore placeholder. (WRITE)
```
enter(item: T, pointerX: number, pointerY: number, index?: number): void {
...
// Don't use items that are being dragged as a reference, because
// their element has been moved down to the bottom of the body.
if (newPositionReference && !this._dragDropRegistry.isDragging(newPositionReference)) {
const element = newPositionReference.getRootElement();
element.parentElement.insertBefore(placeholder, element); // <------ Here WRITE
activeDraggables.splice(newIndex, 0, item);
}
else {
coerceElement(this._element).appendChild(placeholder); // <------ Here WRITE
activeDraggables.push(item);
}
// The transform needs to be cleared so it doesn't throw off the measurements.
placeholder.style.transform = '';
// Note that usually `start` is called together with `enter` when an item goes into a new
// container. This will cache item positions, but we need to refresh them since the amount
// of items has changed.
this._cacheItemPositions(); // <----- HERE READ
}
```
And then this._cacheItemPositions(); which call getMutableClientRect call getBoundingClientRect

Again layout thrashing.
### Suggested Solution
0. let users provide their own implementation DropList Sort Strategy Item extends [DropListSortStrategyItem](https://github.com/angular/components/blob/main/src/cdk/drag-drop/sorting/drop-list-sort-strategy.ts#LL23C35-L23C35)
implements [DropListSortStrategy](https://github.com/angular/components/blob/main/src/cdk/drag-drop/sorting/drop-list-sort-strategy.ts#L35)
Then, dropListRef sortStrategy has to be configured instead of fixed SingleAxisSortStrategy.
```
this._sortStrategy = new SingleAxisSortStrategy(this.element, _dragDropRegistry); // https://github.com/angular/components/blob/main/src/cdk/drag-drop/drop-list-ref.ts#L196
```
1. _cacheItemsPositions, getBoundingClientRect (READ) first, appendChild, extend Styles (WRITE) later. Not sure is it possible.
(I read from [avoid-layout-thrashing thrashing](https://web.dev/avoid-large-complex-layouts-and-layout-thrashing/#avoid-layout-))
2. Same code, but batching read write DOM i.e. [FastDOM](https://github.com/wilsonpage/fastdom)
3. The drag drop library mandatory insert placeholder at startDragSequence which is not optimum. I think placeholder should be optional, and also optional extend styles.
Here is similar Performance profiling in json not same as images above but same problem.
[Profile-20221117T201453.json.zip](https://github.com/angular/components/files/10031835/Profile-20221117T201453.json.zip)
### Reproduction
Steps to reproduce:
1. Production Link to be sent privately
2.
### Expected Behavior
It should have no layout thrashing (no forced reflow).
This image below shown **there is no layout thrashing when I deleted the code insertBefore, extendStyles, but the behavior of dragging is bug.**

### Actual Behavior
There are layout thrashing every time, the placeholder has been inserted.

### Environment
- Angular: 14.2.9
- CDK/Material: CDK
- Browser(s): Chromium (Edge, Chrome), Firefox
- Operating System (e.g. Windows, macOS, Ubuntu): Windows, macOS, Ubuntu
Angular CLI: 14.2.9
Node: 16.13.2
Package Manager: npm 8.5.3
OS: darwin arm64
Angular: 14.2.7
... animations, common, compiler, compiler-cli, core, forms
... google-maps, language-service, platform-browser
... platform-browser-dynamic, router, service-worker
Package Version
------------------------------------------------------------
@angular-devkit/architect 0.1402.6
@angular-devkit/build-angular 14.2.6
@angular-devkit/core 14.2.9
@angular-devkit/schematics 14.2.9
@angular/cdk 14.2.5
@angular/cli 14.2.9
@angular/fire 7.4.1
@angular/material 14.2.5
@angular/material-moment-adapter 14.2.5
@schematics/angular 14.2.9
rxjs 7.5.7
typescript 4.8.4
webpack 5.75.0
### Related issues
angular/angular#20471
angular/components#13372
Contributor guide
Research direction
Start with _toggleNativeDragInteractions and _cacheItemPositions in src/cdk/drag-drop/drag-ref.ts and src/cdk/drag-drop/sorting/single-axis-sort-strategy.ts, using the supplied profiling data and reproduction. Review enter in single-axis-sort-strategy.ts, plus drop-list-sort-strategy.ts and drop-list-ref.ts, to understand the proposed sorting extension. Done means dragging remains correct without the reported forced layout reflows.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- angular, typescript
- Domain
- frontend, performance
- Issue type
- Bug
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 25/100