multi-attach: say who else is watching, and where you are

A small tag in each client's own top-right corner when several
terminals share the session: the viewer count, and on a view that is
a crop being panned by somebody else's cursor, the crop's offset into
the shared screen ("2 clients +12+4") -- which is exactly the moment
a screen moving on its own needs explaining. Stamped per client after
projection, because every terminal has its own count of others and
its own offset; the shared status bar is composed once and can be
cropped out of a panned view anyway. attach_indicator false hides
it.

3 files changed, 41 insertions, 0 deletions

diff --git a/src/app.c b/src/app.c
index 9c9829b..b7af3d1 100644
--- a/src/app.c
+++ b/src/app.c
@@ -93,6 +93,11 @@ bool app_cfg_multi_attach(void) {
   return CFG.multi_attach;
 }
 
+bool app_cfg_attach_indicator(void) {
+  ensure_config();
+  return CFG.attach_indicator;
+}
+
 int app_cfg_size_follows(void) {
   ensure_config();
   return CFG.size_follows;
diff --git a/src/config.c b/src/config.c
index bfbfdf7..9c06983 100644
--- a/src/config.c
+++ b/src/config.c
@@ -919,6 +919,7 @@ void config_defaults(config_t *c) {
   c->status_pad = 4;
   c->focus_follows_mouse = true;
   c->multi_attach = true;
+  c->attach_indicator = true;
   c->size_follows = SIZE_FOLLOWS_ACTIVE;
   c->in_band_shaders = false;
   /* Windows has no VEOF for the line discipline to act on, so ^D there is a
@@ -1613,6 +1614,9 @@ char *config_render(const config_t *c) {
          "multi_attach %s        // several terminals at once; off: a new "
          "client displaces the old\n",
          yesno(c->multi_attach));
+  cb_add(&b,
+         "attach_indicator %s    // top-right tag when others are watching\n",
+         yesno(c->attach_indicator));
   cb_add(&b, "size_follows \"%s\"  // or \"smallest\" / \"largest\"\n",
          c->size_follows == SIZE_FOLLOWS_SMALLEST  ? "smallest"
          : c->size_follows == SIZE_FOLLOWS_LARGEST ? "largest"
@@ -1851,6 +1855,7 @@ const char *config_default_path(void) {
 static const char *const KNOWN_TOP[] = {
     "include", /* read before the rest, but a name this file understands */
     "anim_ms",
+    "attach_indicator",
     "bell_indicator",
     "bell_mark",
     "close_mark",
@@ -2095,6 +2100,8 @@ static bool load_into(config_t *c, const char *path, int depth, char *err,
                                         0, c->focus_follows_mouse);
   c->multi_attach =
       kdl_arg_bool(kdl_child(root, "multi_attach"), 0, c->multi_attach);
+  c->attach_indicator =
+      kdl_arg_bool(kdl_child(root, "attach_indicator"), 0, c->attach_indicator);
   const char *sizef = kdl_arg(kdl_child(root, "size_follows"), 0, NULL);
   if (sizef) {
     if (strcmp(sizef, "active") == 0)
diff --git a/src/server.c b/src/server.c
index 7443648..472fcdd 100644
--- a/src/server.c
+++ b/src/server.c
@@ -308,11 +308,40 @@ static void update_viewport(conn_t *c, const screen_t *src) {
   if (c->shown_rows > c->rows) c->shown_rows = c->rows;
 }
 
+/* The multi-client tag, stamped into one client's projected view: this is
+ * per-client chrome (every terminal has its own count of "others", its own
+ * crop offset), so it cannot live on the shared status bar, which is
+ * composed once and might be cropped out of a panned view anyway. Top-right
+ * of the client's own screen, which is always visible; toasts keep the
+ * bottom-right. Written after projection and before the diff, so it costs
+ * bytes only when it changes -- and when it disappears, the diff restores
+ * the shared cells beneath it on its own. */
+static void stamp_indicator(server_t *s, conn_t *c) {
+  if (!app_cfg_attach_indicator()) return;
+  size_t viewers = display_count(s);
+  if (viewers < 2) return;
+  char tag[48];
+  if (c->view_x || c->view_y)
+    /* The crop's origin in the shared screen: "you are at +col+row of
+     * something bigger", which is the disorienting case the tag exists for. */
+    snprintf(tag, sizeof tag, " %zu clients +%u+%u ", viewers,
+             (unsigned)c->view_x, (unsigned)c->view_y);
+  else
+    snprintf(tag, sizeof tag, " %zu clients ", viewers);
+  size_t len = strlen(tag);
+  if (len >= c->view.cols) return; /* a view too narrow to say it in */
+  uint16_t x0 = (uint16_t)(c->view.cols - len);
+  for (size_t i = 0; i < len; i++)
+    screen_put_utf8(&c->view, (uint16_t)(x0 + i), 0, &tag[i], 1, (color_t){0},
+                    (color_t){0}, ATTR_DIM | ATTR_INVERSE);
+}
+
 static bool push_frame(server_t *s, conn_t *c) {
   uint16_t old_x = c->view_x, old_y = c->view_y;
   uint16_t old_cols = c->shown_cols, old_rows = c->shown_rows;
   update_viewport(c, &s->screen);
   screen_project(&c->view, &s->screen, c->view_x, c->view_y);
+  stamp_indicator(s, c);
   screen_render(&c->view);
   if (!c->view.out_len) return true;
   if (!conn_send(c, MSG_OUTPUT, c->view.out, c->view.out_len)) {