diff --git a/README.md b/README.md
index 261a84c..570a3f5 100644
--- a/README.md
+++ b/README.md
@@ -5,7 +5,7 @@ component hierarchy as a navigation tree, paints the selected folder's children
 wiresheet of draggable component boxes, draws the links between them, and writes box
 positions back into each component's `meta` property on save.
 
-Current version: **0.0.0.048** (see [Versioning](#versioning)).
+Current version: **0.0.0.051** (see [Versioning](#versioning)).
 
 ## Requirements
 
@@ -151,7 +151,7 @@ than leaving the selection looking like it did nothing. So does deleting the com
 is showing, from anywhere -- every delete in the UI goes through `delete_component`,
 because `sax_file` does its own view refresh rather than going through `refresh_all_views`.
 
-It lists **every** slot the manifest declares,
+It lists every slot the manifest declares -- bar one kind, see **Hidden slots** below --
 in manifest order, each row numbered with the same slot id the wiresheet box uses. The
 box's visibility rules do not apply here: they are right for a diagram and wrong for the
 one place a value is meant to be changed, so `HvacControl` shows all 47 rows rather than
@@ -177,10 +177,27 @@ A runtime slot holds a live value that no offline file carries, so nothing here
 usefully write one; a `@readonly` config slot is the manifest saying the same about a
 value that *is* persisted. Both read the same way on screen.
 
+A **dropdown is offered only where there is a choice to make**. A read-only bool or enum
+gets a plain field instead, since a disabled dropdown is an arrow that does nothing -- and
+a read-only bool is still filled by its state, because a runtime bool has a state too.
+
+The dropdown is a `tk.OptionMenu` rather than a `ttk.Combobox`, for one reason: on the
+Windows themes a Combobox draws its field with a native element and **ignores
+`fieldbackground` outright**, so a bool could never be filled with its colour. The plain Tk
+widget takes the colour it is given. It reports a change through its variable rather than
+an event, which also catches a value set from anywhere else -- and means loading a row
+looks exactly like choosing one, so the load is flagged and the tint left alone until every
+row is in.
+
 Each row is drawn from the slot's facets, so it reads the way the kit author meant it to:
 
 - `@trueText` / `@falseText` turn a bool into its own two words -- `Disabled` / `Enabled`
-  rather than false / true -- in a dropdown, with a red or green dot beside it.
+  rather than false / true -- and **the field is filled by the state**: green for true,
+  red for false, blue for null. It began as a coloured dot beside the field, which meant
+  learning what the dot stood for, then as coloured text; a block of colour carries down a
+  long list of slots where a letterform does not, and the words stay black so they read as
+  easily as every other value. Dropping the dot also let a bool's field line up with the
+  numbers above and below it.
 - **Null is a value, not an absence.** Sedona models it as NaN for a float and a sentinel
   for the integer types, and a config slot can genuinely hold it: `control::WriteFloat`
   pairs an `in` property with a `setNull` action, and the sample app carries
@@ -262,6 +279,19 @@ system selection.
 `mark_clean` through a callback `main` registers -- the same shape `undo` uses for its view
 refresh, and what keeps `app_state` free of any idea that a Properties view exists.
 
+### Hidden slots
+
+`@hidden` is the one facet the view filters on, and only by default. niagara.html says
+hidden slots "are designed to be invisible to the user" and that interfaces should rarely
+display them; every one under Sedona home is a `void` action a kit uses internally, such
+as `HdwAI.checkType`. A **Hidden slots (n)** checkbox in the footer brings them back, and
+the answer sticks for the session rather than being asked again for every component.
+
+The rows are always built -- the checkbox `grid_remove`s and re-`grid`s them, which keeps
+each widget's options so putting a row back needs no arithmetic, and an emptied grid row
+collapses rather than leaving a gap. The checkbox itself only appears on a type that has
+something to reveal, so most components carry no control for a thing they do not have.
+
 ### Unsaved edits in the Properties view
 
 An edited field paints the whole view in **`app_state.DIRTY_TINT`** -- the same colour
@@ -431,7 +461,6 @@ Read against `SedonaDoc/niagara.html`, which is what tells Niagara -- and so thi
 
 | Facet | Where it is used | Why not |
 |---|---|---|
-| `hidden` | 129 slots, every one a `void` action such as `HdwAI.checkType` | The doc says user interfaces should rarely display hidden slots, and the reference tool appears not to. This view shows every slot because that is what it is for; say the word and it will skip them. |
 | `fieldWidth` | 3082 slots | A per-slot column count for the text field. Applying it would make the value column ragged; a table reads better with one width. |
 | `radix`, `showSeparators`, `multiLine`, `fieldEditor` | unused by any kit under Sedona home | Nothing to render. |
 
diff --git a/app_state.py b/app_state.py
index 8e3052a..8122ff2 100644
--- a/app_state.py
+++ b/app_state.py
@@ -8,7 +8,7 @@ so later rebinding would only ever be seen by the module that did it.
 import configparser
 import os
 
-VERSION = "0.0.0.048"
+VERSION = "0.0.0.051"
 PROPERTIES_FILE = os.path.join(os.path.dirname(os.path.abspath(__file__)), "editor.properties")
 DEFAULT_GRID_SIZE = 10
 DEFAULT_ADD_TIMESTAMP = True
diff --git a/properties.py b/properties.py
index 86190fb..58f3016 100644
--- a/properties.py
+++ b/properties.py
@@ -5,6 +5,11 @@ table are the same piece of screen and only one of them is ever the thing being
 `main` is what hides the canvas and puts this in its place; this module knows only the
 frame it was handed.
 
+The one slot it does filter is `@hidden`, and only by default: niagara.html says user
+interfaces should rarely display hidden slots, and every one under Sedona home is a `void`
+action a kit uses internally. A checkbox in the footer brings them back, and the choice
+sticks for the session.
+
 The wiresheet box hides most of a type's slots — `HvacControl` paints 8 of its 47 — which
 is right for a diagram and wrong for the one place a value is meant to be changed. This
 view is that place, so it filters nothing: every slot the manifest declares gets a row,
@@ -89,9 +94,21 @@ ROW_BG = "#ffffff"
 HEADER_BG = "#f2f2f2"
 TYPE_COLOUR = "#555555"
 PATH_COLOUR = "#7a7a7a"
-DOT_TRUE = "#3fa34d"
-DOT_FALSE = "#d9534f"
-DOT_NULL = "#4a90d9"
+# A bool says which state it is in by the colour of its field. Filled rather than
+# lettered: a block of colour carries down a long list of slots where coloured text does
+# not, and the words stay black so they read as easily as every other value.
+BOOL_BACKGROUNDS = {
+    "true": "#c8e6c9",
+    "false": "#ffcdd2",
+    "null": "#bbdefb",
+}
+
+
+def bool_state(raw):
+    """Which of the three a value is in: 'true', 'false' or 'null'."""
+    if is_null(raw):
+        return "null"
+    return "true" if str(raw).strip().lower() in ("true", "1") else "false"
 
 FIELD_WIDTH = 18        # characters; the entry and the combobox share it so fields line up
 ROW_HEIGHT_HINT = 22
@@ -100,6 +117,10 @@ ROW_HEIGHT_HINT = 22
 # models null as NaN for a float and a sentinel for the integer types, and a config slot
 # can genuinely hold it: control::WriteFloat pairs an 'in' property with a 'setNull'
 # action, and the sample app carries <prop name="in" val="null"/> on four of them.
+# Whether hidden slots are listed. Held for the session rather than per panel, so the
+# answer does not have to be given again for every component looked at.
+show_hidden_slots = False
+
 NULL_VALUE = "null"
 NULL_TEXT = "null"
 
@@ -221,6 +242,16 @@ def effective_value(comp, slot):
     return type_zero(slot["type"])
 
 
+def is_hidden(slot):
+    """The '@hidden' facet: a slot a kit keeps for itself.
+
+    niagara.html puts it plainly -- hidden slots "are designed to be invisible to the
+    user", and interfaces should rarely display them. Every one under Sedona home is a
+    void action such as `HdwAI.checkType`.
+    """
+    return slot["facets"].get("hidden") is True
+
+
 def is_editable(slot):
     """Config and not read-only. The rule the whole view's greying comes from."""
     return "c" in slot["flags"] and not slot["readonly"]
@@ -470,6 +501,10 @@ class PropertiesPanel:
         # different thing from whether it has been.
         self._tint_targets = []
         self.dirty = False
+        # Set while rows are being filled from the document. An OptionMenu reports through
+        # its variable, so loading a value looks exactly like choosing one; without this
+        # the view would open already claiming unsaved edits.
+        self._loading = False
 
         comp_type = comp.get("type", "sys::Component")
         self.slots = schema.get_full_slots_for_type(comp_type)
@@ -481,8 +516,12 @@ class PropertiesPanel:
         # sibling nothing at all.
         self._build_header(comp_type)
         self._build_footer()
+        self._loading = True
         self._build_rows_area()
+        self._loading = False
+        self._apply_hidden_filter()
         self._load_path()
+        self._update_tint()
 
         if not self.slots:
             self.status_var.set(
@@ -505,7 +544,7 @@ class PropertiesPanel:
         for row in self.rows:
             if not row["editable"]:
                 continue
-            if (row["widget"].get() or "").strip() != row["loaded_text"].strip():
+            if (self.row_text(row) or "").strip() != row["loaded_text"].strip():
                 return True
 
         if self.meta_row is not None:
@@ -518,6 +557,8 @@ class PropertiesPanel:
 
     def _update_tint(self):
         """Repaints the view in the same colour the canvas wears when the file is dirty."""
+        if self._loading:
+            return
         was_dirty = self.dirty
         self.dirty = self.has_unsaved_changes()
         for widget, normal_background in self._tint_targets:
@@ -532,15 +573,42 @@ class PropertiesPanel:
         if was_dirty != self.dirty and self.on_dirty_change is not None:
             self.on_dirty_change()
 
-    def _watch_for_edits(self, widget, kind):
-        """Makes a field repaint the view as soon as its value moves."""
+    def _watch_for_edits(self, row, widget, kind):
+        """Makes a field repaint the view as soon as its value moves.
+
+        `kind` is the widget's, not the slot's: an editable bool is a dropdown, a
+        read-only one is a plain field, and the two report a change differently.
+        """
         if kind == "text":
             widget.bind("<KeyRelease>", lambda _event: self._update_tint(), add="+")
             widget.bind("<<Paste>>", lambda _event: self._update_tint(), add="+")
             widget.bind("<FocusOut>", lambda _event: self._update_tint(), add="+")
+            return
+
+        # An OptionMenu reports through its variable rather than an event, which also
+        # catches a value set from anywhere else.
+        row["var"].trace_add("write", lambda *_args: self._on_choice_changed(row))
+
+    def _on_choice_changed(self, row):
+        # A bool's fill is its value, so picking the other one has to recolour the field
+        # straight away rather than waiting for the next load.
+        if row["kind"] == "bool":
+            self._recolour_bool(row)
+        self._update_tint()
+
+    def _recolour_bool(self, row):
+        """Fills a bool row's field by which of the three states it now reads as."""
+        slot = row["slot"]
+        text = (self.row_text(row) or "").strip()
+        false_text, true_text = bool_texts(slot)
+        if text == true_text:
+            state = "true"
+        elif text == false_text:
+            state = "false"
         else:
-            widget.bind("<<ComboboxSelected>>",
-                        lambda _event: self._update_tint(), add="+")
+            state = "null"
+        fill = BOOL_BACKGROUNDS[state]
+        row["widget"].configure(bg=fill, activebackground=fill)
 
     # -- construction --
 
@@ -639,62 +707,97 @@ class PropertiesPanel:
         dot.grid(row=grid_row, column=0, padx=(6, 2))
         return dot
 
+    def _apply_hidden_filter(self):
+        """Takes hidden rows out of the grid, or puts them back.
+
+        grid_remove keeps each widget's options, so putting a row back needs no
+        arithmetic, and an emptied grid row collapses to nothing rather than leaving a
+        gap where the slot was.
+        """
+        for row in self.rows:
+            if not is_hidden(row["slot"]):
+                continue
+            for widget in row["widgets"]:
+                if show_hidden_slots:
+                    widget.grid()
+                else:
+                    widget.grid_remove()
+
+        self.scroll_canvas.configure(scrollregion=self.scroll_canvas.bbox("all"))
+
     def _build_slot_row(self, grid_row, slot):
-        self._add_bullet(grid_row)
+        bullet = self._add_bullet(grid_row)
 
-        self._tinted(tk.Label(self.rows_frame, text=self._row_label(slot), anchor="w",
-                              bg=ROW_BG, fg=LABEL_COLOUR),
-                     ROW_BG).grid(row=grid_row, column=1, sticky="w", padx=(2, 8))
+        name_label = self._tinted(tk.Label(self.rows_frame, text=self._row_label(slot),
+                                           anchor="w", bg=ROW_BG, fg=LABEL_COLOUR),
+                                  ROW_BG)
+        name_label.grid(row=grid_row, column=1, sticky="w", padx=(2, 8))
 
-        self._tinted(tk.Label(self.rows_frame, text=qualified_type(slot["type"]),
-                              anchor="w", bg=ROW_BG, fg=TYPE_COLOUR),
-                     ROW_BG).grid(row=grid_row, column=2, sticky="w", padx=(0, 10))
+        type_label = self._tinted(tk.Label(self.rows_frame,
+                                           text=qualified_type(slot["type"]),
+                                           anchor="w", bg=ROW_BG, fg=TYPE_COLOUR), ROW_BG)
+        type_label.grid(row=grid_row, column=2, sticky="w", padx=(0, 10))
 
         field_frame = self._tinted(tk.Frame(self.rows_frame, bg=ROW_BG), ROW_BG)
         field_frame.grid(row=grid_row, column=3, sticky="w", padx=(0, 4))
 
         editable = is_editable(slot)
         row = {"slot": slot, "editable": editable, "kind": None, "widget": None,
-               "state_dot": None, "loaded_text": ""}
+               "loaded_text": "",
+               "widgets": [bullet, name_label, type_label, field_frame]}
 
         names = enum_range(slot)
         if slot["type"] == "bool":
             row["kind"] = "bool"
-            state_dot = self._tinted(tk.Canvas(field_frame, width=14, height=16,
-                                               highlightthickness=0, bg=ROW_BG), ROW_BG)
-            state_dot.grid(row=0, column=0)
-            row["state_dot"] = state_dot
-            false_text, true_text = bool_texts(slot)
-            choices = [false_text, true_text]
-            # '@allowNull false' is the manifest saying this bool is only ever one of the
-            # two, so null is offered on every other bool and withheld here.
-            if allows_null(slot):
-                choices.append(null_text(slot))
-            widget = ttk.Combobox(field_frame, values=choices,
-                                  width=FIELD_WIDTH - 3, state="readonly")
-            widget.grid(row=0, column=1)
         elif names and slot["type"] in INTEGER_TYPES:
             row["kind"] = "enum"
-            widget = ttk.Combobox(field_frame, values=list(names),
-                                  width=FIELD_WIDTH - 1, state="readonly")
-            widget.grid(row=0, column=1)
         else:
             row["kind"] = "text"
+
+        # A dropdown is offered only where there is a choice to make: a disabled one on a
+        # runtime slot is an arrow that does nothing.
+        row["is_choice"] = editable and row["kind"] in ("bool", "enum")
+
+        if row["is_choice"]:
+            if row["kind"] == "bool":
+                false_text, true_text = bool_texts(slot)
+                choices = [false_text, true_text]
+                # '@allowNull false' is the manifest saying this bool is only ever one of
+                # the two, so null is offered on every other bool and withheld here.
+                if allows_null(slot):
+                    choices.append(null_text(slot))
+            else:
+                choices = list(names)
+
+            # An OptionMenu rather than a ttk.Combobox, for one reason: the themed
+            # Combobox draws its field with a native element and ignores fieldbackground
+            # outright, so a bool could never be filled with its colour. This is a plain
+            # Tk widget and takes the colour it is given.
+            row["var"] = tk.StringVar()
+            widget = tk.OptionMenu(field_frame, row["var"], *choices)
+            widget.configure(width=FIELD_WIDTH - 5, anchor="w", relief="solid", bd=1,
+                             highlightthickness=0, padx=4, pady=0,
+                             bg=EDITABLE_FIELD_BG, activebackground=EDITABLE_FIELD_BG)
+            widget["menu"].configure(bg=EDITABLE_FIELD_BG)
+        else:
+            row["var"] = None
             widget = tk.Entry(field_frame, width=FIELD_WIDTH, relief="solid", bd=1,
                               highlightthickness=0, justify="left",
                               readonlybackground=READONLY_FIELD_BG)
             widget.configure(bg=EDITABLE_FIELD_BG if editable else READONLY_FIELD_BG)
-            widget.grid(row=0, column=1)
+        widget.grid(row=0, column=1)
 
         row["widget"] = widget
         if editable:
-            self._watch_for_edits(widget, row["kind"])
+            self._watch_for_edits(row, widget,
+                                  "choice" if row["is_choice"] else "text")
 
         suffix = " ".join(part for part in (unit_symbol(slot["facets"].get("unit")),
                                             range_text(slot)) if part)
-        self._tinted(tk.Label(self.rows_frame, text=suffix, anchor="w", bg=ROW_BG,
-                              fg="#555555"),
-                     ROW_BG).grid(row=grid_row, column=4, sticky="w", padx=(6, 6))
+        suffix_label = self._tinted(tk.Label(self.rows_frame, text=suffix, anchor="w",
+                                             bg=ROW_BG, fg="#555555"), ROW_BG)
+        suffix_label.grid(row=grid_row, column=4, sticky="w", padx=(6, 6))
+        row["widgets"].append(suffix_label)
 
         self.rows.append(row)
         self._load_row(row)
@@ -745,6 +848,19 @@ class PropertiesPanel:
         # the tint has to put back.
         self._tinted(footer, footer.cget("bg"))
 
+        # Offered only where there is something to reveal, so most components carry no
+        # control for a thing they do not have.
+        hidden_count = sum(1 for slot in self.slots if is_hidden(slot))
+        if hidden_count:
+            self.show_hidden_var = tk.BooleanVar(value=show_hidden_slots)
+            box = tk.Checkbutton(
+                footer, variable=self.show_hidden_var,
+                text=f"Hidden slots ({hidden_count})",
+                command=self._on_show_hidden_toggled,
+            )
+            self._tinted(box, box.cget("bg"))
+            box.pack(side=tk.LEFT, padx=(0, 12))
+
         self.status_var = tk.StringVar(value="")
         status = tk.Label(footer, textvariable=self.status_var, anchor="w", fg="#333333")
         self._tinted(status, status.cget("bg"))
@@ -757,11 +873,34 @@ class PropertiesPanel:
         ttk.Button(footer, text="Refresh", width=9,
                    command=self.refresh).pack(side=tk.RIGHT)
 
+    def _on_show_hidden_toggled(self):
+        """The checkbox: remember the answer for the session, then re-filter the rows."""
+        global show_hidden_slots
+        show_hidden_slots = bool(self.show_hidden_var.get())
+        self._apply_hidden_filter()
+
     # -- loading --
 
-    def _set_state_dot(self, canvas_widget, colour):
-        canvas_widget.delete("all")
-        canvas_widget.create_oval(3, 4, 12, 13, fill=colour, outline="#666666")
+    def row_text(self, row):
+        """What a row currently reads, whichever kind of field it wears."""
+        if row["var"] is not None:
+            return row["var"].get()
+        return row["widget"].get()
+
+    def _set_row_text(self, row, text):
+        if row["var"] is not None:
+            row["var"].set(text)
+            return
+        widget = row["widget"]
+        widget.configure(state="normal")
+        widget.delete(0, tk.END)
+        widget.insert(0, text)
+
+    def _field_background(self, row, raw):
+        """The fill a row's field takes: a bool's state, or its editability."""
+        if row["kind"] == "bool":
+            return BOOL_BACKGROUNDS[bool_state(raw)]
+        return EDITABLE_FIELD_BG if row["editable"] else READONLY_FIELD_BG
 
     def _load_row(self, row):
         """Puts the DOM's current value into one row's widget and remembers it as loaded."""
@@ -770,28 +909,21 @@ class PropertiesPanel:
         text = display_text(slot, raw)
         widget = row["widget"]
 
-        if row["kind"] == "text":
-            widget.configure(state="normal")
-            widget.delete(0, tk.END)
-            widget.insert(0, text)
-            if row["editable"]:
-                widget.configure(state="normal", fg=LABEL_COLOUR, bg=EDITABLE_FIELD_BG)
-            else:
-                widget.configure(state="readonly", fg=READONLY_TEXT)
-        else:
-            widget.configure(state="readonly")
-            widget.set(text)
-            if not row["editable"]:
-                widget.configure(state="disabled")
+        # Set before the fill, so a StringVar trace sees the value it is colouring for.
+        self._set_row_text(row, text)
+        fill = self._field_background(row, raw)
 
-        if row["state_dot"] is not None:
-            if is_null(raw):
-                colour = DOT_NULL
-            elif str(raw).strip().lower() in ("true", "1"):
-                colour = DOT_TRUE
+        if row["is_choice"]:
+            widget.configure(bg=fill, activebackground=fill)
+            widget["menu"].configure(bg=EDITABLE_FIELD_BG)
+        else:
+            # A bool keeps its fill whether or not it can be changed: the colour is what
+            # says which state the slot is in, and a runtime bool has a state too.
+            widget.configure(fg=LABEL_COLOUR if row["editable"] else READONLY_TEXT)
+            if row["editable"]:
+                widget.configure(state="normal", bg=fill)
             else:
-                colour = DOT_FALSE
-            self._set_state_dot(row["state_dot"], colour)
+                widget.configure(state="readonly", readonlybackground=fill)
 
         row["loaded_text"] = text
 
@@ -902,8 +1034,11 @@ class PropertiesPanel:
 
     def refresh(self):
         """Re-reads every value from the DOM, discarding edits that were not saved."""
+        self._loading = True
         for row in self.rows:
             self._load_row(row)
+        self._loading = False
+
         if self.meta_row is not None:
             self._load_meta_row()
         self._load_path()
@@ -932,7 +1067,7 @@ class PropertiesPanel:
             if not row["editable"]:
                 continue
             slot = row["slot"]
-            current = (row["widget"].get() or "").strip()
+            current = (self.row_text(row) or "").strip()
             if current == row["loaded_text"].strip():
                 continue