Fix forward deletion of the first prompt character

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Unreleased
 
+- Bugfix: a forward deletion of the first prompt character now works instead of dinging. `eca-chat--key-pressed-deletion` guarded the prompt boundary by rejecting any deletion at the prompt-field start regardless of direction, so `C-d` and, under evil, the `~` (`evil-invert-char`), `r`, `x` and `s` operators left the first char in place (`~` prepended the inverted char instead of replacing it). At the prompt-field start the guard now blocks only backward-deleting commands (`delete-backward-char`, `backward-kill-word`, `evil-delete-backward-char`, ...); deletions above the prompt (context/progress area) and everywhere else are unchanged.
 - Show the shell/git command breakdown in tool call approvals: raw command plus per-command lines colored by remember state (green saved, yellow not yet), and `Accept and remember` lists the command keys it would newly save (e.g. `git status, rg`), hiding when nothing new would be remembered.
 
 - Support TRAMP / remote hosts: the eca server starts on the remote host, the binary is resolved from the remote `PATH`, and paths are translated both ways using mappings derived from TRAMP workspace folders (explicit `eca-local-to-remote-prefix-map` entries win). Auto-install is local only: install `eca` on the remote or point `eca-custom-command` at it. #270

eca-chat.el

@@ -1857,6 +1857,23 @@ scrolling is suppressed so the view does not jump."
   (-some-> (eca-chat--prompt-context-field-ov)
     (overlay-start)))
 
+(defconst eca-chat--prompt-boundary-guard-commands
+  '(delete-backward-char
+    backward-delete-char
+    backward-delete-char-untabify
+    backward-kill-word
+    evil-delete-backward-word
+    evil-delete-back-to-indentation
+    evil-delete-backward-char)
+  "Backward-deleting commands blocked at the prompt-field start.
+At the start itself, `eca-chat--key-pressed-deletion' dings only for
+these commands (and for a negative delete count).  Forward deletions
+there - `delete-char' with a positive count, as used by
+`evil-invert-char' (~), `evil-replace' (r), `evil-delete-char' (x) and
+`evil-substitute' (s) - remove the first prompt character and fall
+through to the wrapped command.  Above the prompt field the guard is
+unconditional.")
+
 (defun eca-chat--key-pressed-deletion (side-effect-fn &rest args)
   "Apply SIDE-EFFECT-FN with ARGS before point.
 Unless at the prompt field boundary.
@@ -1896,9 +1913,19 @@ the prompt/context line."
           (setf (nth 2 args) nil) ;; do not delete prompt line passing nil argument
           (apply side-effect-fn args))
 
-         ;; start of the prompt
+         ;; start of the prompt - keep the guard everywhere above the
+         ;; prompt-field start, but at the start itself block only *backward*
+         ;; deletions.  A forward deletion there (`delete-char' from
+         ;; evil-invert-char ~, evil-replace r, evil-delete-char x,
+         ;; evil-substitute s, or C-d) removes the first prompt char, which
+         ;; is legitimate; everything else is unchanged.  A negative count
+         ;; (e.g. `C-u - C-d') makes `delete-char' delete backward, so still
+         ;; guard that.
          ((and prompt-ov
-               (or (<= (point) (overlay-start prompt-ov))
+               (or (< (point) (overlay-start prompt-ov))
+                   (and (= (point) (overlay-start prompt-ov))
+                        (or (memq this-command eca-chat--prompt-boundary-guard-commands)
+                            (and (integerp (car args)) (< (car args) 0))))
                    (and (eq 'backward-kill-word this-command)
                         (string-blank-p (buffer-substring-no-properties
                                          (overlay-start prompt-ov) (point))))))

test/eca-chat-test.el

@@ -133,7 +133,31 @@ does not treat the first line as metadata.  Returns FN's value."
                 (expect side-effect-called :to-be nil)
                 (expect (eca-chat-test--prompt-text buf)
                         :to-equal "hello")))
-          (kill-buffer buf))))))
+          (kill-buffer buf))))
+
+    (it "allows forward deletion at prompt field start"
+      ;; Regression: a forward `delete-char' at the prompt start - plain C-d
+      ;; and, under evil, `evil-invert-char' (~), `evil-replace' (r),
+      ;; `evil-delete-char' (x) and `evil-substitute' (s) - must remove the
+      ;; first prompt char.  The boundary guard used to block it too, so ~
+      ;; prepended the inverted char instead of replacing it in place.
+      (dolist (cmd '(delete-char evil-invert-char evil-replace
+                     evil-delete-char evil-substitute))
+        (let ((buf (eca-chat-test--make-prompt-buffer "hello")))
+          (unwind-protect
+              (with-current-buffer buf
+                (goto-char (eca-chat--prompt-field-start-point))
+                (let ((this-command cmd)
+                      (side-effect-called nil))
+                  (eca-chat--key-pressed-deletion
+                   (lambda (n &optional _)
+                     (setq side-effect-called t)
+                     (delete-char n))
+                   1)
+                  (expect side-effect-called :to-be t)
+                  (expect (eca-chat-test--prompt-text buf)
+                          :to-equal "ello")))
+            (kill-buffer buf)))))))
 
 (describe "eca-chat--protect-non-prompt"