fix: Consistent single blank line between chat sections (#98)

Two fixes for extra blank lines in chat buffer:

1. display-agent-end: normalize to single trailing \n (was \n\n).
   Section headers prepend \n, so \n\n created triple newlines.

2. display-message-delta: strip leading newlines from first delta.
   Models send \n\n before first content, creating blank lines
   after the setext header. Detected by comparing markers: when
   message-start-marker equals streaming-marker, no content has
   been inserted yet.

Author: dnouri

pi-coding-agent.el

@@ -960,6 +960,12 @@ on each delta - fontification happens at message end instead."
   (when (and delta pi-coding-agent--streaming-marker)
     (let* ((inhibit-read-only t)
            (inhibit-modification-hooks t)
+           ;; Strip leading newlines from first content after header
+           (delta (if (and pi-coding-agent--message-start-marker
+                          (= (marker-position pi-coding-agent--message-start-marker)
+                             (marker-position pi-coding-agent--streaming-marker)))
+                     (string-trim-left delta "\n+")
+                   delta))
            (transformed (pi-coding-agent--transform-delta delta)))
       (pi-coding-agent--with-scroll-preservation
         (save-excursion
@@ -1008,7 +1014,7 @@ CONTENT is ignored - we use what was already streamed."
           (set-marker pi-coding-agent--streaming-marker (point)))))))
 
 (defun pi-coding-agent--display-agent-end ()
-  "Display end of agent turn and process follow-up queue.
+  "Finalize agent turn: normalize whitespace, handle abort, process queue.
 Note: status is set to `idle' by the event handler."
   ;; Reset local message tracker.
   ;; Ensures clean state for next turn (e.g., if abort occurred before echo arrived).
@@ -1030,14 +1036,13 @@ Note: status is set to `idle' by the event handler."
             (insert "\n\n" (propertize "[Aborted]" 'face 'error) "\n")))
         (setq pi-coding-agent--aborted nil)
         (setq pi-coding-agent--followup-queue nil))
-      ;; Add spacing for next turn, avoiding excess blank lines
-      ;; Use scroll preservation so following windows stay at end
+      ;; Normalize trailing newlines - ensure content ends with single newline.
       (pi-coding-agent--with-scroll-preservation
         (save-excursion
           (goto-char (point-max))
           (skip-chars-backward "\n")
           (delete-region (point) (point-max))
-          (insert "\n\n"))))
+          (insert "\n"))))
     (pi-coding-agent--spinner-stop)
     (pi-coding-agent--fontify-timer-stop)
     (pi-coding-agent--refresh-header)

test/pi-coding-agent-test.el

@@ -773,13 +773,13 @@ as the top-level structure."
     (pi-coding-agent--display-thinking-delta " about this.")
     (should (string-match-p "Let me think... about this." (buffer-string)))))
 
-(ert-deftest pi-coding-agent-test-display-agent-end-adds-newlines ()
-  "agent_end event adds trailing newlines."
+(ert-deftest pi-coding-agent-test-display-agent-end-adds-newline ()
+  "agent_end normalizes trailing whitespace to single newline."
   (with-temp-buffer
     (pi-coding-agent-chat-mode)
     (pi-coding-agent--append-to-chat "Some response")
     (pi-coding-agent--display-agent-end)
-    (should (string-suffix-p "\n\n" (buffer-string)))))
+    (should (string-suffix-p "response\n" (buffer-string)))))
 
 (ert-deftest pi-coding-agent-test-spacing-no-blank-line-after-user-header ()
   "User header has no blank line after setext underline.
@@ -800,6 +800,26 @@ The hidden === provides visual spacing when `markdown-hide-markup' is t."
     ;; Pattern: setext heading (Assistant + underline), NO blank line, content
     (should (string-match-p "Assistant\n=+\nHi" (buffer-string)))))
 
+(ert-deftest pi-coding-agent-test-spacing-delta-leading-newlines-stripped ()
+  "Leading newlines from first text delta are stripped.
+Models often send \\n\\n before first content, which would create
+extra blank lines after the setext header."
+  (with-temp-buffer
+    (pi-coding-agent-chat-mode)
+    (pi-coding-agent--display-agent-start)
+    (pi-coding-agent--display-message-delta "\n\nHi")
+    (should (string-match-p "Assistant\n=+\nHi" (buffer-string)))))
+
+(ert-deftest pi-coding-agent-test-spacing-thinking-leading-newlines-stripped ()
+  "Leading newlines before thinking block are stripped.
+Models may send \\n\\n before thinking content too."
+  (with-temp-buffer
+    (pi-coding-agent-chat-mode)
+    (pi-coding-agent--display-agent-start)
+    (pi-coding-agent--display-thinking-start)
+    ;; Thinking start should appear directly after header, no blank line
+    (should (string-match-p "Assistant\n=+\n>" (buffer-string)))))
+
 (ert-deftest pi-coding-agent-test-spacing-blank-line-before-tool ()
   "Tool block is preceded by blank line when after text."
   (with-temp-buffer
@@ -823,6 +843,34 @@ The hidden === provides visual spacing when `markdown-hide-markup' is t."
     ;; Should end with closing fence and blank line
     (should (string-match-p "```\n\n" (buffer-string)))))
 
+(ert-deftest pi-coding-agent-test-spacing-single-blank-line-between-turns ()
+  "Only one blank line between agent response and next section header.
+agent_end + next section's leading newline must not create triple newlines."
+  (with-temp-buffer
+    (pi-coding-agent-chat-mode)
+    ;; Turn 1: user + assistant
+    (pi-coding-agent--display-user-message "Hi")
+    (pi-coding-agent--display-agent-start)
+    (pi-coding-agent--display-message-delta "Hello!")
+    (pi-coding-agent--render-complete-message)
+    (pi-coding-agent--display-agent-end)
+    ;; Turn 2: user message
+    (setq pi-coding-agent--assistant-header-shown nil)
+    (pi-coding-agent--display-user-message "Bye")
+    ;; Should never have triple newlines (which would be two blank lines)
+    (should-not (string-match-p "\n\n\n" (buffer-string)))))
+
+(ert-deftest pi-coding-agent-test-spacing-single-blank-line-before-compaction ()
+  "Only one blank line between agent response and compaction header."
+  (with-temp-buffer
+    (pi-coding-agent-chat-mode)
+    (pi-coding-agent--display-agent-start)
+    (pi-coding-agent--display-message-delta "Some response.")
+    (pi-coding-agent--render-complete-message)
+    (pi-coding-agent--display-agent-end)
+    (pi-coding-agent--display-compaction-result 50000 "Summary.")
+    (should-not (string-match-p "\n\n\n" (buffer-string)))))
+
 (ert-deftest pi-coding-agent-test-spacing-no-double-blank-between-tools ()
   "Consecutive tools have single blank line between them."
   (with-temp-buffer