RFR: 8306121: Scene not rendered initially when changing scenes after fix for JDK-8296621 [v11]

[Thiago Milczarek Sayao]

> I replicated the fix on #1054 on Linux.
> 
> Also fixes [Monocle: Stage no longer gets focus after fix for JDK-8296621](https://bugs.openjdk.org/browse/JDK-8304734)

Thiago Milczarek Sayao has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 53 additional commits since the last revision:

 - Merge branch 'master' into change_scene_fix
 - Fix ComboBoxTest
 - Add manual test
 - Merge branch 'master' into change_scene_fix
 - Merge branch 'openjdk:master' into master
 - Merge branch 'master' into change_scene_fix
 - Merge remote-tracking branch 'origin/master'
 - Review changes
 - Merge branch 'master' into change_scene_fix
 - Change call to requestFocus
 - ... and 43 more: https://git.openjdk.org/jfx/compare/08e5f00e...7828c2a9

-------------

Changes:
  - all: https://git.openjdk.org/jfx/pull/1110/files
  - new: https://git.openjdk.org/jfx/pull/1110/files/c394214c..7828c2a9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1110&range=10
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1110&range=09-10

  Stats: 2016 lines in 32 files changed: 1767 ins; 160 del; 89 mod
  Patch: https://git.openjdk.org/jfx/pull/1110.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1110/head:pull/1110

PR: https://git.openjdk.org/jfx/pull/1110