diff --git a/README.md b/README.md
index dc86b7b..6b94772 100644
--- a/README.md
+++ b/README.md
@@ -2403,7 +2403,9 @@ now — they are listed so they are not mistaken for anything else:
   accessible name. Both panes also have a **locked** mode, which is what a returning user gets: the
   trigger pane shows one line saying what it is set to, the signal list shows only the signals that are
   on, and neither offers a control that changes the configuration — each heading has an `[edit]` button
-  to switch to the full pane, and back. A **first run** (no session file for that source) comes up
+  to switch to the full pane, and back. Dragging a signal onto a plot still works while locked — filling
+  plots is what the list is for, not configuration — and a locked row can be dragged from anywhere on it,
+  the colour chip included. A **first run** (no session file for that source) comes up
   unlocked, because configuring the host is what a first run is for. Each pane's heading also carries its **own switch**, the same control as the
   header bar's button — clicking either moves both — because a pane that is *disabled* while its
   feature is off is a pane whose controls you would otherwise have to leave to turn it back on. The
diff --git a/docs/claude/invariants-session-and-export.md b/docs/claude/invariants-session-and-export.md
index f1ba76a..9d8b4e3 100644
--- a/docs/claude/invariants-session-and-export.md
+++ b/docs/claude/invariants-session-and-export.md
@@ -18,7 +18,11 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   a control that changes the configuration. A *first run* — no session file for that source — never
   reaches `apply_layout()` and stays in configuration mode, which is what a first run is for. The mode
   is a mode and not a setting: it is deliberately not persisted, so a scope reopened tomorrow starts
-  where a returning user expects to start.
+  where a returning user expects to start. **Dragging a row onto a plot is not configuration and survives
+  the lock**: filling plots is what the list is *for*, so a locked row keeps its `GtkDragSource` — from
+  anywhere on it, the colour chip included, which in configuration mode has a click gesture that claims
+  the sequence precisely so a drag cannot start there. `drag_sources` on the `signals panel:` line is the
+  reading, and it must be `rows - 1`.
 - **The session is the layout.** `LayoutState` persists what makes a reopened scope look like the
   one that was closed: the window, the rows and their slot signals, the calibration, **the enabled
   signals**, **the chosen trace colours** (`[[color]]`, `name` + `rgb = "#RRGGBB"` — only the
diff --git a/docs/claude/probe-readings.md b/docs/claude/probe-readings.md
index cbd7f43..3245bcd 100644
--- a/docs/claude/probe-readings.md
+++ b/docs/claude/probe-readings.md
@@ -1378,7 +1378,11 @@ the two lines together read what the file gave and what the code put in it.
 The signals panel's frame read from the window, and in the same line the *shape* of the one channel list:
 the heading and the hint the `.ui` gives it, the list the rows go in, how many rows that list holds
 (`rows`), how many of them are streamed signals (`signals`), how many math channels the app has (`math`),
-and `divider`, which is 1 when `rows == signals + 1 + math` and 0 otherwise. `labels=1` is a `.ui` that lost
+and `divider`, which is 1 when `rows == signals + 1 + math` and 0 otherwise. **`drag_sources` must be
+`rows - 1`** — one per signal row, the `MATH` divider not being draggable. Dragging a signal onto a plot is
+how a plot is filled, so it has to survive both pane modes: the locked list drops the tick, the menu and the
+`+`, and this is the reading that says the drag did not go with them. (`0` is a list that has become a thing
+you can read but not use.) `labels=1` is a `.ui` that lost
 one of the two labels — counted over the panel's subtree *above* the list, because the rows' own name
 and unit labels are the code's and the heading sits in a heading row beside the pane's `[edit]` button — `list=0` one that lost the list -- both of which otherwise show as a panel quietly
 missing a piece -- and **`divider=0` is the merged list that lost its `+`**, the one control that can create
diff --git a/src/ui.cpp b/src/ui.cpp
index c941889..1a9e4d3 100644
--- a/src/ui.cpp
+++ b/src/ui.cpp
@@ -5747,12 +5747,13 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
       // ones, and the `MATH` divider plus the math channels account for the rest -- so
       // `rows == signals + 1 + math` is the invariant, and `divider=0` is a list that lost its `+`.
       std::printf("[udpscope]   signals panel: labels=%d list=%d rows=%d signals=%zu math=%zu "
-                  "divider=%d\n",
+                  "divider=%d drag_sources=%d\n",
                   labels, lists, app.dbg_signal_rows, app.dbg_signal_plain, app.math.size(),
                   app.dbg_signal_rows ==
                           static_cast<int>(app.dbg_signal_plain + 1 + app.math.size())
                       ? 1
-                      : 0);
+                      : 0,
+                  app.dbg_signal_drag_sources);
     }
     {
       // The events pane's frame, read from the window: the four widgets the `.ui` gives it, and the list
diff --git a/src/ui_app.hpp b/src/ui_app.hpp
index 5086adb..38ad53e 100644
--- a/src/ui_app.hpp
+++ b/src/ui_app.hpp
@@ -820,6 +820,8 @@ struct App {
   /// of them are streamed signals. The `MATH` divider and the math channels account for the rest, and
   /// `rows == signals + 1 + math` is the invariant the dump checks -- a list that lost its `+`.
   int dbg_signal_rows = 0;
+  /// How many rows of the list carry a `GtkDragSource`: `rows - 1`, the divider not being draggable.
+  int dbg_signal_drag_sources = -1;
   std::size_t dbg_signal_plain = 0;
   int dbg_side_column_after_hide = 1;
   std::string dbg_mode_triggered;
diff --git a/src/ui_probe.cpp b/src/ui_probe.cpp
index f4a7079..aa0f852 100644
--- a/src/ui_probe.cpp
+++ b/src/ui_probe.cpp
@@ -85,6 +85,24 @@ gboolean exit_probe_step3(gpointer user) {
     a->dbg_switcher_w = gtk_widget_get_width(a->side_switcher);
     a->dbg_switcher_h = gtk_widget_get_height(a->side_switcher);
   }
+  // How many rows of the signal list carry a drag source: dragging a signal onto a plot is how a plot is
+  // filled, so it has to survive both pane modes -- the locked list drops the tick, the menu and the `+`,
+  // and this is the reading that says it did not drop the drag with them. One row is the `MATH` divider,
+  // which is not draggable: the count must be `rows - 1`.
+  if (a->signal_list != nullptr) {
+    int sources = 0;
+    for (GtkWidget* r = gtk_widget_get_first_child(a->signal_list); r != nullptr;
+         r = gtk_widget_get_next_sibling(r)) {
+      GListModel* controllers = gtk_widget_observe_controllers(r);
+      const guint n = controllers != nullptr ? g_list_model_get_n_items(controllers) : 0;
+      for (guint i = 0; i < n; ++i) {
+        GObject* c = G_OBJECT(g_list_model_get_item(controllers, i));
+        if (GTK_IS_DRAG_SOURCE(c)) ++sources;
+        g_object_unref(c);
+      }
+    }
+    a->dbg_signal_drag_sources = sources;
+  }
   // The panes' own switches, and *both* directions of the sync: the header bar's trigger button is
   // flipped here and the pane's switch has to follow, then the cursors' switch is driven from the
   // pane and the button is what has to follow. Both are read back after the fact, so the two pairs in