D5515: Update the ungrabMouse hack for Qt 5.8

[David Edmundson]

davidedmundson created this revision.
Restricted Application added projects: Plasma, Frameworks.
Restricted Application added subscribers: Frameworks, plasma-devel.

REVISION SUMMARY
  There was an unidentified bug where when opening a context menu in
  response to a mousePress event led to the next click events not being
  registered. The mouse got grabbed by the item in the mousePressEvent,
  but when we open a menu it didn't get released. The next click goes to
  the currently grabbed item.
  
  A workaround was added back in 2015 (and copied into various other parts
  of Plasma) to ungrab the mouse after launching the context menu inside
  our rightClickEvent. This doesn't work because qquickwindow code is
  shuffled. It now sends the event and then grabs the mouse.
  
  qquickwindow.cpp:653 on Qt5.8.0
  
  Which means we're now trying to do the ungrab, before QWindow has set
  our item as the grabber. That obviously doesn't work.
  
  This real bug needs fixing in Qt, (I now know the root cause!)
  but we have a workaround here already, we may as well make it work.
  
  Note: This also needs doing in a bajillion (6) places in p-w/p-d.
  
  FWIW, the root cause of all of these bugs is:
  
  - QtQuick internally keeps track of which item has "grabbed" the mouse
  - It's weirdly a singleton between windows, but that should be fine
  - It relies on the fact that when we get an X mouse press event we
  
  should always get a mouse release event. X is surprisingly smart and
  will send this even if you change window focus.
  
  - However - context menus are evil bastards. They do a low level grab
  
  (completely different type of grab than the one above) and grab all
  mouse events at the X level
  
  - Our original QtQuickItem which was handling the right click event no
  
  longer gets our release event
  
  - The reason it's only somtimes is we have a "race" between being
  
  created and the grab going through X, and the user actually releasing
  the mouse.

TEST PLAN
  Had 1 reproducible case
  Added this. No longer could reproduce

REPOSITORY
  R242 Plasma Framework (Library)

BRANCH
  origin-master (branched from master)

REVISION DETAIL
  https://phabricator.kde.org/D5515

AFFECTED FILES
  src/scriptengines/qml/plasmoid/containmentinterface.cpp

To: davidedmundson, #plasma
Cc: plasma-devel, #frameworks, spstarr, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, sebas, apol
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