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

[Akib Azmain Turja <akib@disroot.org>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> From: Akib Azmain Turja <akib@disroot.org>
>> Date: Fri, 08 Jul 2022 18:40:16 +0600
>> 
>> 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.
>
> Try this:
>
>   https://tronche.com/gui/x/xlib/window/attributes/gravity.html
>

This seems like to answer I found, except the terms are different.

If I understand correctly, the following happen when gravity is
CenterGravity:

--8<---------------cut here---------------start------------->8---
+----------+              +--------+
|   some   |              |  some  |              +------+
| contents | -> resize -> |contents| -> resize -> |ontent|
|   here   |              |  here  |              +------+
+----------+              +--------+
--8<---------------cut here---------------end--------------->8---

Am I correct?

But I can't still understand why Emacs needs this.  Emacs handles frame
resizing, so setting gravity seems useless.

-- 
Akib Azmain Turja

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