Fix derivative sources visibility; code editor monospace + 4-space tabs

- Always insert one source row for derivative/second_derivative on build
- Guard in _on_kind: add row when switching to derivative with no sources
- Code editor: monospace font (IBM Plex Mono/Consolas fallback)
- Code editor: tab stop = 4 character widths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/ui/windows/channels_window.py b/ui/windows/channels_window.py
index d2a9813..a90cff6 100644
--- a/ui/windows/channels_window.py
+++ b/ui/windows/channels_window.py
@@ -28,7 +28,7 @@ from PyQt6.QtWidgets import (
     QToolButton, QSplitter,
 )
 from PyQt6.QtCore import Qt, pyqtSignal
-from PyQt6.QtGui import QCloseEvent
+from PyQt6.QtGui import QCloseEvent, QFont, QFontMetrics
 
 from devices.device_registry import DeviceRegistry
 from core.signal_processor import (
@@ -612,9 +612,12 @@ class DerivedBlock(QFrame):
         self._src_cont = QWidget(); self._src_vlay = QVBoxLayout(self._src_cont)
         self._src_vlay.setContentsMargins(0,0,0,0); self._src_vlay.setSpacing(3)
         _is_deriv = self.dc.kind in ("derivative", "second_derivative")
-        src_init = [self.dc.sources[0]] if (_is_deriv and self.dc.sources) else self.dc.sources
-        for s in src_init:
-            self._add_src(s)
+        if _is_deriv:
+            # Always show exactly one y-source row
+            self._add_src(self.dc.sources[0] if self.dc.sources else None)
+        else:
+            for s in self.dc.sources:
+                self._add_src(s)
         self._add_src_btn = QPushButton("＋ Add Source"); self._add_src_btn.setObjectName("addTraceBtn")
         self._add_src_btn.clicked.connect(lambda: self._add_src())
         src_lay.addWidget(self._src_cont); src_lay.addWidget(self._add_src_btn)
@@ -651,6 +654,9 @@ class DerivedBlock(QFrame):
         code_lay = QVBoxLayout(self._code_grp); code_lay.setContentsMargins(6,4,6,4)
         self._code = QTextEdit(); self._code.setObjectName("codeEditor")
         self._code.setMinimumHeight(90); self._code.setMaximumHeight(180)
+        _mono = QFont("IBM Plex Mono, Consolas, Monospace"); _mono.setStyleHint(QFont.StyleHint.Monospace)
+        self._code.setFont(_mono)
+        self._code.setTabStopDistance(QFontMetrics(_mono).horizontalAdvance(" ") * 4)
         txt = self.dc.expression if self.dc.kind=="expression" else self.dc.script
         self._code.setPlainText(txt)
         code_lay.addWidget(self._code)
@@ -695,6 +701,8 @@ class DerivedBlock(QFrame):
         k = _ALL_KINDS[idx]
         self.dc.kind = k
         self._help.setText(_KIND_HELP.get(k,""))
+        if k in ("derivative", "second_derivative") and not self._src_combos:
+            self._add_src()
         self._update_visibility()
 
     def _on_wrt_changed(self, idx: int):