Fix dirty marker and Reset All button styling issues

- Fix _update_groupbox_dirty_markers to only consider fields under current
  nested manager's prefix instead of all fields in state
- Fix Reset All button styling to use font underline instead of underscore
  character for signature diff indication

Bug fixes for nested form manager dirty state tracking

Author: trissim

src/pyqt_reactive/forms/parameter_form_manager.py

@@ -809,18 +809,32 @@ def _update_reset_button_styling(self, param_name: str) -> None:
         update_reset_button_styling(reset_button, self.state, self.field_id, param_name)
 
     def _update_groupbox_dirty_markers(self) -> None:
-        """Update groupbox dirty markers (title and Reset All button)."""
+        """Update groupbox dirty markers (title and Reset All button).
+
+        CRITICAL: Only consider fields under THIS nested manager's prefix.
+        The state contains ALL fields, so we must filter by field_id prefix.
+        """
         # Find the groupbox widget for this manager
         groupbox = None
         if self._parent_manager:
             for name, nested in self._parent_manager.nested_managers.items():
                 if nested is self:
                     groupbox = self._parent_manager.widgets.get(name)
                     break
-        
+
         if groupbox and hasattr(groupbox, 'set_dirty_marker'):
-            has_dirty = bool(self.state.dirty_fields)
-            has_sig_diff = bool(self.state.signature_diff_fields)
+            # Filter dirty_fields and signature_diff_fields to only include
+            # fields under this nested manager's prefix (field_id)
+            prefix = self.field_id + '.' if self.field_id else ''
+
+            if prefix:
+                has_dirty = any(f.startswith(prefix) for f in self.state.dirty_fields)
+                has_sig_diff = any(f.startswith(prefix) for f in self.state.signature_diff_fields)
+            else:
+                # Root manager - check all fields
+                has_dirty = bool(self.state.dirty_fields)
+                has_sig_diff = bool(self.state.signature_diff_fields)
+
             groupbox.set_dirty_marker(has_dirty, has_sig_diff)
 
     def _update_provenance_button_visibility(self) -> None:

src/pyqt_reactive/widgets/shared/clickable_help_components.py

@@ -1051,16 +1051,18 @@ def set_dirty_marker(self, is_dirty: bool, has_sig_diff: bool = False) -> None:
             self._update_reset_all_button_styling(is_dirty, has_sig_diff)
 
     def _update_reset_all_button_styling(self, is_dirty: bool, has_sig_diff: bool) -> None:
-        """Update Reset All button with * and _ styling."""
+        """Update Reset All button with * and _ styling.
+
+        * prefix indicates dirty (unsaved changes)
+        Underline (via font) indicates signature diff (differs from signature default)
+        """
         button = self._reset_all_button
         text = "Reset All"
         if is_dirty:
             text = "*" + text
-        if has_sig_diff:
-            text = "_" + text
         button.setText(text)
 
-        # Apply underline via font (independent of dirty)
+        # Apply underline via font (NOT as literal underscore character)
         font = button.font()
         font.setUnderline(has_sig_diff)
         button.setFont(font)