fix focustop(): prefer a visible fullscreen client

Problem (reproducible on vanilla 0.9, unrelated to any local patch):
1. fullscreen client A on tag 3 (MOD+e);
2. MOD+0 to view all tags;
3. MOD+1 to switch to tag 1, focus lands on client B;
4. MOD+0 again: client A is displayed fullscreen, covering everything,
   but the focused client is the invisible B;
5. so MOD+e / MOD+q act on B behind A and the user thinks MOD+e does
   not work on A until A is clicked with the mouse.

Root cause: focustop() returns the first VISIBLEON() client on the
fstack, but VISIBLEON() only checks whether the client is on a
currently visible tag; it does not check whether the client is covered
by a fullscreen client. When switching back to tag 0, the last focused
client B is first on the fstack, so B gets the focus while A is still
displayed fullscreen.

Fix: focustop() first looks for a visible fullscreen client on the
fstack (at most one fullscreen client per monitor is actually shown,
and it always sits on top, covering the whole monitor), and only falls
back to the previous "first visible client" if there is none. As a
result:
- after a tag switch the focus goes back to the fullscreen client that
  actually occupies the screen, so MOD+e / MOD+q behave as expected;
- the fullscreen_bg decision in arrange() is fixed as a side effect
  (it used to be turned off when the focus was not on the fullscreen
  client, so a non-opaque fullscreen surface could reveal other
  clients behind it, which violates the protocol);
- focusstack() already returns early when the focused client is
  fullscreen, so MOD+j/k and the arrow keys no longer move the focus
  to a covered client either.

Verified with headless dwl, two foot clients and keys injected through
zwp_virtual_keyboard_v1: before the fix the last step gives
focal = winB / fullscreen 0; after the fix focal = winA / fullscreen 1,
and MOD+e then restores A as expected.

ta: 20261031_022753

README.md

@@ -153,6 +153,7 @@ given the base on which it is built. Implemented default features are:
 - Support screen lockers via ext-session-lock-v1 protocol
 - Various Wayland protocols
 - XWayland support as provided by wlroots (can be enabled in `config.mk`)
+- Zero flickering - Wayland users naturally expect that "every frame is perfect"
 - Layer shell popups (used by Waybar)
 - Damage tracking provided by scenegraph API
 

client.h

@@ -189,6 +189,7 @@ client_get_geometry(Client *c, struct wlr_box *geom)
 	}
 #endif
 	*geom = c->surface.xdg->geometry;
+	geom->x = geom->y = 0;
 }
 
 static inline Client *
@@ -257,48 +258,6 @@ client_is_float_type(Client *c)
 			(min.width == max.width || min.height == max.height));
 }
 
-static inline int
-client_is_rendered_on_mon(Client *c, Monitor *m)
-{
-	/* This is needed for when you don't want to check formal assignment,
-	 * but rather actual displaying of the pixels.
-	 * Usually VISIBLEON suffices and is also faster. */
-	struct wlr_surface_output *s;
-	int unused_lx, unused_ly;
-	if (!wlr_scene_node_coords(&c->scene->node, &unused_lx, &unused_ly))
-		return 0;
-	wl_list_for_each(s, &client_surface(c)->current_outputs, link)
-		if (s->output == m->wlr_output)
-			return 1;
-	return 0;
-}
-
-static inline int
-client_is_stopped(Client *c)
-{
-	int pid;
-	siginfo_t in = {0};
-#ifdef XWAYLAND
-	if (client_is_x11(c))
-		return 0;
-#endif
-
-	wl_client_get_credentials(c->surface.xdg->client->client, &pid, NULL, NULL);
-	if (waitid(P_PID, pid, &in, WNOHANG|WCONTINUED|WSTOPPED|WNOWAIT) < 0) {
-		/* This process is not our child process, while is very unlikely that
-		 * it is stopped, in order to do not skip frames, assume that it is. */
-		if (errno == ECHILD)
-			return 1;
-	} else if (in.si_pid) {
-		if (in.si_code == CLD_STOPPED || in.si_code == CLD_TRAPPED)
-			return 1;
-		if (in.si_code == CLD_CONTINUED)
-			return 0;
-	}
-
-	return 0;
-}
-
 static inline int
 client_is_unmanaged(Client *c)
 {
@@ -365,19 +324,36 @@ client_set_scale(struct wlr_surface *s, float scale)
 	wlr_surface_set_preferred_buffer_scale(s, (int32_t)ceilf(scale));
 }
 
-static inline void
+/* Returns nonzero if the client has to commit a buffer with the new size */
+static inline uint32_t
 client_set_size(Client *c, uint32_t width, uint32_t height)
 {
 #ifdef XWAYLAND
 	if (client_is_x11(c)) {
 		wlr_xwayland_surface_configure(c->surface.xwayland,
 				c->geom.x + c->bw, c->geom.y + c->bw, width, height);
-		return;
+		return (int)width != client_surface(c)->current.width
+				|| (int)height != client_surface(c)->current.height;
 	}
 #endif
-	if ((int32_t)width != c->surface.xdg->toplevel->current.width
-			|| (int32_t)height != c->surface.xdg->toplevel->current.height)
-		wlr_xdg_toplevel_set_size(c->surface.xdg->toplevel, (int32_t)width, (int32_t)height);
+	if ((int32_t)width == c->surface.xdg->toplevel->scheduled.width
+			&& (int32_t)height == c->surface.xdg->toplevel->scheduled.height)
+		return 0;
+	return wlr_xdg_toplevel_set_size(c->surface.xdg->toplevel, (int32_t)width, (int32_t)height);
+}
+
+static inline void
+client_send_frame_done_iterator(struct wlr_surface *s, int sx, int sy, void *data)
+{
+	wlr_surface_send_frame_done(s, data);
+}
+
+static inline void
+client_send_frame_done(Client *c)
+{
+	struct timespec now;
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	wlr_surface_for_each_surface(client_surface(c), client_send_frame_done_iterator, &now);
 }
 
 static inline void

config.def.h

@@ -12,7 +12,6 @@ static const float rootcolor[]             = COLOR(0x222222ff);
 static const float bordercolor[]           = COLOR(0x444444ff);
 static const float focuscolor[]            = COLOR(0x005577ff);
 static const float urgentcolor[]           = COLOR(0xff0000ff);
-/* This conforms to the xdg-protocol. Set the alpha to zero to restore the old behavior */
 static const float fullscreen_bg[]         = {0.0f, 0.0f, 0.0f, 1.0f}; /* You can also use glsl colors */
 
 /* tagging - TAGCOUNT must be no greater than 31 */
@@ -172,6 +171,9 @@ static const Button buttons[] = {
 };
 
 static const Axis axes[] = {
+	/* example of volume control:
 	{ MODKEY, AxisUp,   spawn, SHCMD("volume-up_EXAMPLE") },
-	{ MODKEY, AxisDown, spawn, SHCMD("volume-down_EXAMPLE") },
+	{ MODKEY, AxisDown, spawn, SHCMD("volume-down_EXAMPLE") }, */
+	{ 0, 0, NULL, {0} },
+	/* does nothing, but the array cannot be empty */
 };

config.mk

@@ -1,4 +1,4 @@
-_VERSION = 0.9-dev
+_VERSION = 0.10-dev
 VERSION  = `git describe --tags --dirty 2>/dev/null || echo $(_VERSION)`
 
 PKG_CONFIG = pkg-config

ime.h

@@ -594,7 +594,7 @@ handle_focused_surface_destroy(struct wl_listener *listener,
 }
 
 struct input_method_relay *
-input_method_relay_create()
+input_method_relay_create(void)
 {
 	struct input_method_relay *relay = ecalloc(1, sizeof(*relay));
 	wl_list_init(&relay->text_inputs);