So, after a quick IRC chat with timp, let me recap what happens in the
gallery:

1) When the layout is switched to 1 column, Gallery sets
header.flickable=templateFlickable so that the header hides and shows
based on user interaction with the page

2) If the user scrolls down, that hides the header (exposed = false)

3) When the layout is switched to 2 columns, Gallery sets
header.flickable=null so that the header is locked when in 2 panels
mode, BUT if the user has not touched the surface after 2) then the
header will still have expose=false (i.e it's scrolled out of the view).
Since flickable is now null, the header will not show based on user
interaction, thus causing the bug.

So, we have 2 solutions here, as timp pointed out on IRC:
1) App developer sets exposed=true when he also sets flickable= null
OR
2) Header components automatically sets exposed=true when its "flickable" 
property is set to null, to prevent cases like the one described by this bug 
from happening

The latter is a behaviour change, and it is definitely useful and likely
the most common usecase, but maybe there are usecases where the
developer may want to have more control over what happens to the Header
after the hide-and-show-based-on-interaction mechanism is disabled (by
setting flickable=null)

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https://bugs.launchpad.net/bugs/1556860

Title:
  Blank left panel header in gallery after resizing AdaptivePageLayout
  from two panels to one and back

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