AdaptivePageLayout breaks on loading heavy pages

[Michael Zanetti]

When the pages to be loaded by AdaptivePageLayout are heavy (say, require a second to build up), the APL, because of his async nature by default, still allows user interaction while creating the page.

If during that time the user triggers another action that would push something to the APL, it will break in two ways:
* the first page will leak in memory
* the second page is visible on the apl, but the back button in the header is missing

setting APL to asynchronous: false is a workaround for this as the user cannot trigger any more actions while the page is building up.

If you need an example to reproduce this, install "Guitar Tools" from the store, edit its sources to remove the "asynchronous: false" from the APL in there and launch the composer tool and quickly another entry from the menu. The composer tool is the one that takes long to load, any other entry can be used to trigger the bug.

I have also seen reports from users in other apps that the back button in the header is missing sometimes. I am confident they are running into the same issue.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.