Bug 260443 – Kinetic scrolling stops after elements are slotted

• Status: NEW
• Component: Scrolling

[Steve Orvell]

Created attachment 467346 [details]
Scrolling freezes after scrolling horizontally inside the top bordered container on iOS/iPadOS

Setup: 
1. An element has a shadowRoot with a <slot> in it.
2. The element is scrollable.
3. A child element is appended and slotted to the <slot> *while* the element is scrolling. 

Expected: 
* The element continues to scroll after the child is appended.
Actual: 
* The element immediately stops scrolling after the child is appended.

Reproduction:

* Scroll horizontally inside each bordered container.
* Note, this issue occurs only on iOS/iPadOS.

The attached reproduction shows the *incorrect* behavior when a child is appended and slotted, and it also shows the *expected* behavior when a child is appended to the shadowRoot itself rather than slotted. The reproduction is also available at https://lit.dev/playground/#gist=05deaacf5ba04c069cc41b9d523b636b&view-mode=preview.

[Simon Fraser (smfr)]

Does this happen with equivalent DOM mutations that don't involve slots?

[Steve Orvell]

No, when the mutation does not involve slotting, scrolling works as expected. See the explanation of the repro at the end of the initial report.

[Radar WebKit Bug Importer]

<rdar://problem/114523840>