iOS/Safari: "Select all" results in empty selection on large documents

[personalizedrefrigerator]

Describe the issue

On sufficiently large documents, pressing "select all" in the iOS pop-up context menu moves the cursor, rather than selecting.

Screen recording:

RPReplay_Final1701752181.mov

Browser and platform

iOS 17.1.2

Reproduction link

https://codemirror.net/try/?c=aW1wb3J0IHttaW5pbWFsU2V0dXAsIEVkaXRvclZpZXd9IGZyb20gImNvZGVtaXJyb3IiCgpjb25zdCBsaW5lcyA9IFtdCgpmb3IgKGxldCBpID0gMDsgaSA8IDUwMDsgaSsrKSB7CiAgbGluZXMucHVzaChgTGluZSAke2l9YCkKfQoKbmV3IEVkaXRvclZpZXcoewogIGRvYzogbGluZXMuam9pbignXG4nKSwKICBleHRlbnNpb25zOiBtaW5pbWFsU2V0dXAsCiAgcGFyZW50OiBkb2N1bWVudC5ib2R5Cn0pCg==

[marijnh]

When the viewport doesn't cover the entire document, it seems the height placeholders rendered instead of the hidden content confuse Safari a lot—it refuses to select anything when they are uneditable. When made editable, it still doesn't select them for some reason. Attached patches add kludges to make this work.