D5438: [plugins/qpa] Do not replace a valid FBO with an invalid one

[Martin Gräßlin]

graesslin created this revision.
Restricted Application added a project: KWin.
Restricted Application added subscribers: kwin, plasma-devel.

REVISION SUMMARY
  With the TabBox I observed the following issue:
   1: trigger TabBox through screen edge
   2: exit TabBox
   3: trigger TabBox again through screen edge
  
  Actual behavior: in step 3 the GUI does not show. If now using Alt+Tab
  to show the TabBox the gui shows, but through screen edge it stays
  broken.
  
  Investigation showed that the difference is a direct show in screen edge
  case and a delayed show in Alt+Tab case. Futher investigation shows that
  an invalid geometry gets requested in the broken case. While this might
  indicate an issue in another area it makes sense to protect KWin
  internally against it and not to break rendering.
  
  Thus this change ensures that a valid FBO does not get replaced by an
  invalid sized FBO.

TEST PLAN
  Tested that Alt+Tab works correctly when triggered through
  edge

REPOSITORY
  R108 KWin

BRANCH
  qpa-fbo-valid

REVISION DETAIL
  https://phabricator.kde.org/D5438

AFFECTED FILES
  plugins/qpa/window.cpp

To: graesslin, #kwin, #plasma
Cc: plasma-devel, kwin, progwolff, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol
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