RFR: 8292922 - [Linux] No more drag events when new Stage is created in drag handler [v7]

[Thiago Milczarek Sayao]

> This PR  was previously discussed on #905.
> 
> The approach is to grab the keyboard focus so the window that originated the drag will keep it.
> 
> I did some cleanup on grabbing related functions as well.
> 
> `gdk_keyboard_focus()` is deprecated, so is `gdk_device*` functions in favor of `gdk_seat*`. But that's only available in later Gtk versions. I checked and newer Gtk will use `gdk_seat*` inside the deprecated `gdk_keyboard_focus()`.
> 
> Edit:
> 
> The current changes uses another approach that is to not ungrab pointer device when focus is received on another window. There's also some cleanup on grabbing related functions and grab moved from starting on mouse click to the actual drag.

Thiago Milczarek Sayao has updated the pull request incrementally with one additional commit since the last revision:

  Only grab once drag starts

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/977/files
  - new: https://git.openjdk.org/jfx/pull/977/files/35c45b09..996c214c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=06
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=977&range=05-06

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/977.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/977/head:pull/977

PR: https://git.openjdk.org/jfx/pull/977