Added Desktop Switch Triggers for different animations

This adds support for desktop switching animations by keeping track of _NET_CURRENT_DESKTOP atom on the root window. 

As far as I understand this atom is set by window managers and so if it changes we can know that it's a desktop switch happening. Unfortunately window manager may need to set this BEFORE hiding/showing windows so we can animate correctly, me personally using FluxBox and this quick change pijulius/fluxbox@83ee4db  makes it work just fine.

It adds the following animation triggers:
* workspace-out
* workspace-out-inverse
* workspace-in
* workspace-in-inverse

Unfortunately had to add inverse variables too as you may navigate to the next workspace from for e.g. 1st to 2nd but you may also go to 2nd from 1st and in that case the animations have to be totally different.

Here is a config example for switching workspace:

animations = ({
    triggers = ["workspace-out"];
    offset-y = {
        timing = "0.2s cubic-bezier(0.21, 0.02, 0.76, 0.36)";
        start = "0";
        end = "-window-height";
    };
    shadow-offset-y = "offset-y";
    opacity = {
        timing = "0.2s linear";
        start = "window-raw-opacity-before";
        end = "window-raw-opacity";
    };
    blur-opacity = "opacity";
    shadow-opacity = "opacity";
},
{
    triggers = ["workspace-out-inverse"];
    offset-y = {
        timing = "0.2s cubic-bezier(0.21, 0.02, 0.76, 0.36)";
        start = "0";
        end = "window-height + window-y";
    };
    shadow-offset-y = "offset-y";
    opacity = {
        timing = "0.2s linear";
        start = "window-raw-opacity-before";
        end = "window-raw-opacity";
    };
    blur-opacity = "opacity";
    shadow-opacity = "opacity";
},
{
    triggers = ["workspace-in"];
    offset-y = {
        timing = "0.2s cubic-bezier(0.24, 0.64, 0.79, 0.98)";
        start = "window-height + window-y";
        end = "0";
    };
    shadow-offset-y = "offset-y";
    opacity = {
        timing = "0.2s linear";
        start = "0";
        end = "window-raw-opacity";
    };
    blur-opacity = "opacity";
    shadow-opacity = "opacity";
},
{
    triggers = ["workspace-in-inverse"];
    offset-y = {
        timing = "0.2s cubic-bezier(0.24, 0.64, 0.79, 0.98)";
        start = "-window-height";
        end = "0";
    };
    shadow-offset-y = "offset-y";
    opacity = {
        timing = "0.2s linear";
        start = "0";
        end = "window-raw-opacity";
    };
    blur-opacity = "opacity";
    shadow-opacity = "opacity";
})

src/atom.h

@@ -25,7 +25,8 @@
 	_NET_WM_WINDOW_TYPE, \
 	_XROOTPMAP_ID, \
 	ESETROOT_PMAP_ID, \
-	_XSETROOT_ID
+	_XSETROOT_ID, \
+	_NET_CURRENT_DESKTOP
 
 #define ATOM_LIST2 \
 	_NET_WM_WINDOW_TYPE_DESKTOP, \

src/common.h

@@ -184,6 +184,10 @@ typedef struct session {
 	int root_width;
 	/// Height of root window.
 	int root_height;
+	/// Current desktop number of root window
+	int root_desktop_num;
+	/// Desktop switch direction
+	int root_desktop_switch_direction;
 	/// X Composite overlay window.
 	xcb_window_t overlay;
 	/// The target window for debug mode

src/config.h

@@ -95,7 +95,11 @@ enum animation_trigger {
 	ANIMATION_TRIGGER_OPEN,
 	/// When a window is closed
 	ANIMATION_TRIGGER_CLOSE,
-	ANIMATION_TRIGGER_LAST = ANIMATION_TRIGGER_CLOSE,
+	ANIMATION_TRIGGER_WORKSPACE_IN,
+	ANIMATION_TRIGGER_WORKSPACE_IN_INVERSE,
+	ANIMATION_TRIGGER_WORKSPACE_OUT,
+	ANIMATION_TRIGGER_WORKSPACE_OUT_INVERSE,
+	ANIMATION_TRIGGER_LAST = ANIMATION_TRIGGER_WORKSPACE_OUT_INVERSE,
 };
 
 static const char *animation_trigger_names[] attr_unused = {
@@ -105,6 +109,10 @@ static const char *animation_trigger_names[] attr_unused = {
     [ANIMATION_TRIGGER_DECREASE_OPACITY] = "decrease-opacity",
     [ANIMATION_TRIGGER_OPEN] = "open",
     [ANIMATION_TRIGGER_CLOSE] = "close",
+    [ANIMATION_TRIGGER_WORKSPACE_IN] = "workspace-in",
+    [ANIMATION_TRIGGER_WORKSPACE_IN_INVERSE] = "workspace-in-inverse",
+    [ANIMATION_TRIGGER_WORKSPACE_OUT] = "workspace-out",
+    [ANIMATION_TRIGGER_WORKSPACE_OUT_INVERSE] = "workspace-out-inverse",
 };
 
 struct script;

src/event.c

@@ -591,6 +591,17 @@ static inline void ev_property_notify(session_t *ps, xcb_property_notify_event_t
 	}
 
 	if (ps->c.screen_info->root == ev->window) {
+		// If desktop number property changes
+		if (ev->atom == ps->atoms->a_NET_CURRENT_DESKTOP) {
+			auto prop = x_get_prop(&ps->c, ps->c.screen_info->root, ps->atoms->a_NET_CURRENT_DESKTOP,
+						1L, XCB_ATOM_CARDINAL, 32);
+
+			if (prop.nitems) {
+				ps->root_desktop_switch_direction = ((int)*prop.c32) - ps->root_desktop_num;
+				ps->root_desktop_num = (int)*prop.c32;
+			}
+		}
+
 		if (ps->o.use_ewmh_active_win && ps->atoms->a_NET_ACTIVE_WINDOW == ev->atom) {
 			// to update focus
 			ps->pending_updates = true;

src/picom.c

@@ -1822,6 +1822,9 @@ static void draw_callback_impl(EV_P_ session_t *ps, int revents attr_unused) {
 	ps->next_render = 0;
 	ps->render_queued = false;
 
+	// Reset workspace switching so correct animations are used afterward
+	ps->root_desktop_switch_direction = 0;
+
 	// TODO(yshui) Investigate how big the X critical section needs to be. There are
 	// suggestions that rendering should be in the critical section as well.
 
@@ -2281,6 +2284,16 @@ static session_t *session_init(int argc, char **argv, Display *dpy,
 		goto err;
 	}
 
+	// Initiate current workspace num so we correctly animate if picom 
+	// started/restarted on different desktop number than 0
+	auto prop = x_get_prop(&ps->c, ps->c.screen_info->root, ps->atoms->a_NET_CURRENT_DESKTOP,
+				1L, XCB_ATOM_CARDINAL, 32);
+
+	ps->root_desktop_switch_direction = 0;
+	if (prop.nitems) {
+		ps->root_desktop_num = (int)*prop.c32;
+	}
+
 	rebuild_screen_reg(ps);
 
 	bool compositor_running = false;

src/win.c

@@ -2309,6 +2309,15 @@ void win_process_animation_and_state_change(struct session *ps, struct managed_w
 		case WSTATE_PAIR(WSTATE_UNMAPPED, WSTATE_MAPPED):
 			trigger = w->in_openclose ? ANIMATION_TRIGGER_OPEN
 			                          : ANIMATION_TRIGGER_SHOW;
+
+			if (ps->root_desktop_switch_direction != 0) {
+				trigger = 
+					(ps->root_desktop_switch_direction < 0 &&
+					 ps->root_desktop_switch_direction >= -1) ||
+					 ps->root_desktop_switch_direction > 1 ?
+						ANIMATION_TRIGGER_WORKSPACE_IN_INVERSE  :
+						ANIMATION_TRIGGER_WORKSPACE_IN;
+			}
 			break;
 		case WSTATE_PAIR(WSTATE_UNMAPPED, WSTATE_DESTROYED):
 			if ((!ps->o.no_fading_destroyed_argb || !win_has_alpha(w)) &&
@@ -2326,6 +2335,15 @@ void win_process_animation_and_state_change(struct session *ps, struct managed_w
 			break;
 		case WSTATE_PAIR(WSTATE_MAPPED, WSTATE_UNMAPPED):
 			trigger = ANIMATION_TRIGGER_HIDE;
+
+			if (ps->root_desktop_switch_direction != 0) {
+				trigger = 
+					(ps->root_desktop_switch_direction < 0 &&
+					 ps->root_desktop_switch_direction >= -1) ||
+					 ps->root_desktop_switch_direction > 1 ?
+						ANIMATION_TRIGGER_WORKSPACE_OUT_INVERSE :
+						ANIMATION_TRIGGER_WORKSPACE_OUT;
+			}
 			break;
 		default:
 			log_error("Impossible state transition from %d to %d", old_state,