[css-overflow] which interactions can cause scrolling with hidden scroll containers #6355
Description
For overflow:hidden scroll containers, as well as (since #4113) for scroll or auto scroll containers with visibility: hidden and descendants that have visibility: visible, the spec say that programmatic scrolling is allowed, while direct interactive scrolling such as with the mouse wheel is not.
But there doesn't seem to be full agreement about the precise set of things that can/cannot trigger scrolling. There are disagreements between browsers, and disagreements within a single browser on visibility: hidden vs overflow: hidden:
| Chrome | Safari | Firefox | ||
|---|---|---|---|---|
| mouse wheel / touch-pad scroll / touch-screen drag | `overflow:hidden` | ❌ | ❌ | ❌ |
| `visibility:hidden` | ❌ | ✅ (agreed to switch) | ✅ (agreed to switch) | |
| arrow / page-up / page-down / … keys on the scroll container | `overflow:hidden` | ❌ | ❌ | ❌ |
| `visibility:hidden` | ❌ | ❌ | ❌ | |
| arrow / page-up / page-down / … keys on a descendant | `overflow:hidden` | ❌ | ❌ | ❌ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| tab-focusing a descendant | `overflow:hidden` | ✅ | ✅ | ✅ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| navigating to the fragment identifier of a descendant | `overflow:hidden` | ✅ | ✅ | ✅ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| Extending a selection with the mouse | `overflow:hidden` | ❌ | ❌ | ❌ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| Extending a selection with the keyboard | `overflow:hidden` | ✅ | ✅ | ❌ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| Moving the caret / inserting text in a contenteditable element | `overflow:hidden` | ✅ | ✅ | ✅ |
| `visibility:hidden` | ✅ | ✅ | ✅ | |
| Search in page | `overflow:hidden` | ❌ | ✅ | ❌ |
| `visibility:hidden` | ✅ | ✅ | ✅ |
(results established by playing around with http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=9378)
Presumably, given the resolution in #4113 (comment), we want to get rid of the differences between overflow:hidden and visibility:hidden, but which behavior should we pick for each case? What's the higher level principle?