Bug 983991 - Ensure the pagination bar layer is always here. r=crdlc
Bug 983991 - Move the will-change rule from ol to .page. r=crdlc
Bug 983991 - Do not reflows on panels changes. r=crdlc
Bug 983991 - Keep panels partly on screen after transition to preserve their layers. r=crdlc
Merge pull request #17248 from vingtetun/bug983991.keep.layers

Bug983991.keep.layers
Revert "Merge pull request #15692 from 6a68/settings-app-WIP-rebased"

This reverts commit 8328ac15d589c213373c0849cfcff9a34e715a25, reversing
changes made to c149d0fe8cb3ca08d28ae1d94d9ee9147be52e68.