Implement on-the-spot IME support for X11

[raulsntos]

Implements the linux part of #53658

Currently the area with the options is rendered in the wrong position:

[raulsntos]

I don't think this is supposed to work.

The list of supported XIMStyles did not contain any with XIMPreeditCallbacks for me, I was only getting XIMPreeditPosition which is supposed to be over-the-spot (not on-the-spot), forcing it seems to work.

[raulsntos]

The window_set_ime_position method does not seem to work with the XIMPreeditCallbacks style, the XSetICValues method returns a non-null value so I guess that doesn't work.

I don't know how I'm supposed to update the position of the suggestion list when using XIMPreeditCallbacks.

godot/platform/linuxbsd/display_server_x11.cpp

Lines 1818 to 1830 in 0eed8d8

	::XPoint spot;
	spot.x = short(p_pos.x);
	spot.y = short(p_pos.y);
	XVaNestedList preedit_attr = XVaCreateNestedList(0, XNSpotLocation, &spot, nullptr);
	{
	// Block events polling during this call
	// because it triggers some event polling internally.
	MutexLock mutex_lock(events_mutex);
	XSetICValues(wd.xic, XNPreeditAttributes, preedit_attr, nullptr);
	}
	XFree(preedit_attr);

[voylin]

Thanks for your work on this, any idea when this may finally get reviewed so it can be added to the next update?

[raulsntos]

The suggestion list still shows in the wrong position and I could not figure out why, so until that's resolved this won't be merged since it offers no benefit over the current implementation that uses over-the-spot.

If anyone more knowledgeable about X11 knows what I'm doing wrong I'd really appreciate the help.

[salty-godzilla]

I'm not familiar with X Input Method so I've been investigating for a few days, But unfortunately XIM may not have a way to specify a candidate position when using on-the-spot style. https://gist.github.com/garasubo/8ebb893f2af1950254df715fcf1de58e#gistcomment-4012210

In SDL2, the candidates are correctly displayed (at least in my Fcitx environment) at the position specified by SDL_SetTextInputRect (sample). But it looks like it is communicating directly with Fcitx (code) and IBus (code) through DBus...

[voylin]

I've been looking up some info about this, but it is a quite a bit above my level. I hope somebody will be able to do this by the time the beta's start rolling out.

[bruvzg]

I'm not familiar with X Input Method so I've been investigating for a few days, But unfortunately XIM may not have a way to specify a candidate position when using on-the-spot style.

If it's displayed under the window, I guess we can try adding an invisible sub-window as a host for IME, add use its position to move candidate window.

[raulsntos]

I've tried to create a sub_window to use for IME but then the sub_window steals the focus and I can't give back focus to the main window because it seems when I try to focus back on the main window the LineEdit still has the focus and reopens the IME sub_window, maybe I'm doing it wrong.

For now, I rebased but I'm marking this PR as a draft and moving the milestone to 4.x since I don't think I'll finish this for 4.0. But if anyone wants to take over and implement this sooner then go ahead.

[voylin]

I may give this a try, not certain if I can succeed or not.
Don't have that much time so if I don't come up with anything within this month then I probably failed.

[raulsntos]

Superseded by #70052