[C source] What is gravity and the change_gravity argument?

[Akib Azmain Turja]

In src/termhooks.h, I found the following two members of struct
terminal:

--8<---------------cut here---------------start------------->8---
  /* This hook is called to change the size of frame F's native
   (underlying) window.  If CHANGE_GRAVITY, change to top-left-corner
   window gravity for this size change and subsequent size changes.
   Otherwise we leave the window gravity unchanged.  */
  void (*set_window_size_hook) (struct frame *f, bool change_gravity,
                                int width, int height);

  /* CHANGE_GRAVITY is 1 when calling from Fset_frame_position,
   to really change the position, and 0 when calling from
   *_make_frame_visible (in that case, XOFF and YOFF are the current
   position values).  It is -1 when calling from gui_set_frame_parameters,
   which means, do adjust for borders but don't change the gravity.  */

  void (*set_frame_offset_hook) (struct frame *f, register int xoff,
                                 register int yoff, int change_gravity);
--8<---------------cut here---------------end--------------->8---

X and PGTK uses that 'change_gravity' argument, I can't understand what
they are doing with them.  I searched the internet and found several
answers[1], but couldn't make any sense.


Footnotes:
[1]  https://stackoverflow.com/q/3482742/11819469

-- 
Akib Azmain Turja

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