multi-attach: the config knobs

The patch made shared attach unconditional; these make it a policy.
multi_attach false restores the classic displacing attach
(EXIT_REPLACED, which old clients already understand). size_follows
picks whose size the shared screen takes: active (the patch's rule),
smallest (the tmux rule: filler, never a crop), or largest. The size
fold lives in apply_canvas, so any attach, resize or departure can
move the canvas under the folding policies -- including a non-active
client leaving, which under smallest may have been the very client
holding the floor down. A config reload re-applies both knobs to the
running session.

2 files changed, 24 insertions, 0 deletions

diff --git a/include/app.h b/include/app.h
index 92f2f85..b3dd57c 100644
--- a/include/app.h
+++ b/include/app.h
@@ -59,6 +59,11 @@ bool app_reload_config(char *err, size_t errcap);
 /* The files the config in force was read from: the one that was loaded and
  * everything it included, existing or not. What the watcher watches. */
 size_t app_config_files(const char **out, size_t max);
+/* The attach-sharing knobs, read fresh so a config reload changes policy on
+ * a running session: may several clients attach at once, and whose size the
+ * shared screen takes (SIZE_FOLLOWS_* in config.h). */
+bool app_cfg_multi_attach(void);
+int app_cfg_size_follows(void);
 /* What the config in force complained about while loading, or "". A complaint is
  * not a failure -- an include that is not there, a shader nobody has heard of --
  * so the session is running either way; this is for saying so. */
diff --git a/include/config.h b/include/config.h
index fee6b2e..9db9c53 100644
--- a/include/config.h
+++ b/include/config.h
@@ -112,6 +112,13 @@ typedef struct {
 
 typedef enum { ALIGN_LEFT, ALIGN_CENTER, ALIGN_RIGHT } align_t;
 
+/* size_follows: which attached client sizes the shared screen. */
+enum {
+  SIZE_FOLLOWS_ACTIVE = 0, /* newest attach, then last input */
+  SIZE_FOLLOWS_SMALLEST,   /* min over clients: filler, never a crop */
+  SIZE_FOLLOWS_LARGEST,    /* max over clients: crops, never filler */
+};
+
 /* How many files one config may be built from: itself plus its includes. Small
  * and fixed, because a config assembled from more than this many pieces is a
  * thing nobody can read. */
@@ -306,6 +313,18 @@ typedef struct {
   uint16_t status_pad;
   bool focus_follows_mouse;
 
+  /* Several terminals may attach to one session at once (the default).
+   * false restores the classic rule: a new client displaces the one before
+   * it, which is what anyone using attach-from-elsewhere as "move my session
+   * here and *close it there*" actually wants. */
+  bool multi_attach;
+  /* Whose size the shared screen takes when several clients are attached:
+   * the active client (newest attach, then last input -- the default), or
+   * the smallest / largest attached one. Smallest is the tmux rule: nobody
+   * is cropped and the roomiest terminal pays in filler; largest gives the
+   * big display everything and small ones a cursor-following crop. */
+  int size_follows;
+
   /* Whether Ctrl-D at an idle shell prompt exits that shell, the way the tty
    * line discipline would do it on POSIX.
    *