After r235153, I can't move the selection start grabber when selecting text in a subframe on iOS, if the subframe isn't at the very top of the main document. To reproduce:
1. Visit https://whsieh.github.io/examples/subframes
2. Scroll to the very bottom, where "lorem-tiny.html" is embedded in a subframe
3. Long press to select some text
4. Try to move the grabber at the start of the selection.
Expected: able to change the selection by moving the grabber.
Observed: the selection start grabber snaps to the character before the selection end grabber, and cannot be moved.
Created attachment 349268 [details]
Comment on attachment 349268 [details]
Clearing flags on attachment: 349268
Committed r235834: <https://trac.webkit.org/changeset/235834>
All reviewed patches have been landed. Closing bug.