Make pty/run and /log streams, and take refused lines back cleanly

Review fixes to pty/run and /log:

- Both are streams with a per-open cursor. A shell's exec 3<>file shares
  one offset between write and read, so cat <&3 after echo make >&3 asked
  for offset 5 and got "0" instead of "exit 0"; log after follow lost its
  first bytes the same way.
- A run is accepted only in pardes's own tagged input phase, so a nested
  shell's prompt (ssh, a shell with its own integration) is never taken for
  this shell's.
- A line the shell refused is taken back so the next finds the prompt
  clear: bash's continuation prompt (now tagged k=c) gets Ctrl-C and the
  answer waits for the fresh prompt; a bash syntax error (no C, but a D) is
  answered at once; fish's kept line gets Ctrl-U (a Ctrl-C sent while fish
  redraws is lost). fish's right prompt no longer reads as typed input.
- bash marks a command's start from PS0 on bash 4.4+, not a DEBUG trap, so
  a user's own DEBUG trap (bash-preexec, atuin) survives. bash's own job
  notices now show as in plain bash (ttyfollow golden).
- 32 run slots; a new line on an open starts a fresh answer.

Checked live in bash, fish, and fish with a right prompt.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 18 insertions, 5 deletions

diff --git a/src/panes.zig b/src/panes.zig
index 3b1bb1b4..410492b8 100644
--- a/src/panes.zig
+++ b/src/panes.zig
@@ -6520,6 +6520,12 @@ pub const Terminal = struct {
         /// Counts prompts drawn (B), so a line the shell refused without
         /// running (a fish syntax error: no C, no D, a new prompt) shows.
         prompts: u32 = 0,
+        /// D marks outside a running command: the shell handled a line
+        /// without starting it (bash on a syntax error prints no C).
+        declined: u32 = 0,
+        /// The newest prompt continues an unfinished line (bash's PS2, marked
+        /// `k=c`): the shell still holds the start of the command.
+        continuation: bool = false,
         /// The newest command to end: which one, and its status if given.
         finished: ?struct { seq: u32, status: ?i32 } = null,
 
@@ -6530,7 +6536,10 @@ pub const Terminal = struct {
                 const aid = value.readOption(.aid) orelse break :mark;
                 if (!std.mem.eql(u8, aid, prompt_aid)) break :mark;
                 switch (value.action) {
-                    .fresh_line_new_prompt => self.phase = .prompt,
+                    .fresh_line_new_prompt => {
+                        self.phase = .prompt;
+                        self.continuation = value.readOption(.prompt_kind) == .continuation;
+                    },
                     .end_prompt_start_input => {
                         self.phase = .input;
                         self.prompts +%= 1;
@@ -6544,6 +6553,8 @@ pub const Terminal = struct {
                     .end_command => if (self.phase == .running) {
                         self.phase = .none;
                         self.finished = .{ .seq = self.started, .status = value.readOption(.exit_code) };
+                    } else {
+                        self.declined +%= 1;
                     },
                     else => {},
                 }
@@ -7346,14 +7357,16 @@ pub const Terminal = struct {
             state.vt.screens.active.cursor.semantic_content == .input;
     }
 
-    /// At a prompt with nothing typed on its line, wherever the cursor is
-    /// (someone may have typed and pressed Ctrl-A). fish suggests nothing on
-    /// an empty line, so its autosuggestions never make this false.
+    /// At a prompt with nothing typed on its line: no input left of the
+    /// cursor and none under it (text typed and then Ctrl-A'd starts at the
+    /// cursor). Nothing further right counts: that is where fish draws a
+    /// right prompt, and fish suggests nothing on an empty line.
     pub fn promptInputEmpty(pane: *const Pane) bool {
         if (comptime !enabled) return false;
         if (!promptInputReady(pane)) return false;
         const cursor = &pane.terminal.?.vt.screens.active.cursor;
-        for (cursor.page_pin.cells(.all)) |cell|
+        const row = cursor.page_pin.cells(.all);
+        for (row[0..@min(cursor.x + 1, row.len)]) |cell|
             if (cell.semantic_content == .input and !cell.isEmpty()) return false;
         return true;
     }