panes: read and write a pane by id, without taking focus

`raw` wrote into the focused pane and `snapshot` composited the active tab,
so driving a pane anywhere else meant focusing it or selecting its tab
first. Both of those move the view of whoever is watching, and both lose to
anybody doing the same thing at the same time.

`raw` now takes an `id` (0 is the focused pane, as before), and `capture`
answers one pane's viewport as text -- from the pane's own terminal, so it
works for a pane in a background tab, changes no focus, and leaves the
human's selection alone. It builds its selection as a snapshot rather than
setting one on the terminal, because a read with a visible side effect is
not a read.

`capture` tells "no such pane" apart from "that pane is empty": a caller
polling a dead id would otherwise wait forever on a pane that is quiet
because it is gone.

1 files changed, 49 insertions, 0 deletions

diff --git a/src/pane.c b/src/pane.c
index 4c5da85..d1b4f66 100644
--- a/src/pane.c
+++ b/src/pane.c
@@ -1264,6 +1264,55 @@ char *pane_selection_text(pane_t *p) {
   return out;
 }
 
+/* The pane's whole viewport as text: what somebody looking at this pane would
+ * read, for a pane in any tab, without touching anything.
+ *
+ * A snapshot of the *session* is the composited screen and only the active tab
+ * is composited, so reading one pane used to mean bringing its tab up, slicing
+ * its rect out of the frame and putting the tab back -- which moves the view
+ * of whoever is watching, and races anybody else doing the same. This asks the
+ * pane's own terminal instead.
+ *
+ * The selection is built here and handed to the formatter as a snapshot rather
+ * than set on the terminal: formatting the *terminal's* selection would mean
+ * overwriting whatever the human had selected, so a read would have a visible
+ * side effect. `unwrap` is off, unlike a copy: this answers "what is on
+ * screen", and a row is a line there even when it is the tail of a longer one.
+ *
+ * NULL when there is nothing (a pane whose terminal has no viewport yet).
+ * Caller frees. */
+char *pane_viewport_text(pane_t *p) {
+  if (!p || !p->term || !p->cols || !p->rows_n) return NULL;
+  GhosttyGridRef a, b;
+  if (!grid_ref_at(p, 0, 0, &a) ||
+      !grid_ref_at(p, (uint16_t)(p->cols - 1), (uint16_t)(p->rows_n - 1), &b))
+    return NULL;
+
+  GhosttySelection sel = GHOSTTY_INIT_SIZED(GhosttySelection);
+  sel.start = a;
+  sel.end = b;
+  sel.rectangle = false;
+
+  GhosttyTerminalSelectionFormatOptions opts =
+      GHOSTTY_INIT_SIZED(GhosttyTerminalSelectionFormatOptions);
+  opts.emit = GHOSTTY_FORMATTER_FORMAT_PLAIN;
+  opts.unwrap = false;
+  opts.trim = true;
+  opts.selection = &sel;
+
+  uint8_t *ptr = NULL;
+  size_t len = 0;
+  if (ghostty_terminal_selection_format_alloc(p->term, NULL, opts, &ptr,
+                                              &len) != GHOSTTY_SUCCESS ||
+      !len)
+    return NULL;
+  char *out = malloc(len + 1);
+  memcpy(out, ptr, len);
+  out[len] = 0;
+  ghostty_free(NULL, ptr, len);
+  return out;
+}
+
 /* ---- scrollback --------------------------------------------------------- */
 
 void pane_scroll(pane_t *p, int delta) {