fix: the sidebar keeps its own width, so a small window can no longer hide it

The sidebar's width was a GtkPaned divider position: `window - 940` in pixels. On any window
narrower than 940 px the divider landed past the right edge and the sidebar came up 0 px wide,
reachable only by dragging the divider back.

Every attempt that kept the paned failed for that reason -- deriving the position from the paned's
own width, re-parenting the column, property-form children, code-set size requests. The mechanism was
the fault, so it is gone: the row is an AdwOverlaySplitView, which is what a utility sidebar is in
GNOME, with min-sidebar-width = max-sidebar-width = 320 in pixels. Its width is therefore its own at
any window size, and when there is no room beside the plot it *overlays* the plot (collapsed, set from
the window's width in tick()) instead of being squeezed to nothing.

Two things found on the way, both recorded in the invariants:

- sidebar-width-fraction is a 0..1 fraction of the window whatever sidebar-width-unit says. Setting a
  pixel count there is a GObject critical and it falls back to 0.25, which is why the sidebar first
  read 560. The width is pinned with min/max instead.
- the window opened at its content minimum (756 px, the plot column plus the sidebar) because the
  explicit gtk_window_set_default_size() was dropped when the window moved into the .ui. Restored.

[sidebar-probe] reads both states and is the guard: window=756 sidebar=320 under a 2x scale, which is
the case that used to read 0.

docs/claude/invariants-plot-and-layout.md

@@ -4,6 +4,22 @@ How `plot::Figure`/`plot::Plot` may and may not be driven, what a row's state be
 
 Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to learn; read the ones touching the area you are editing before you edit it.
 
+- **A sidebar's size must be its own, never a divider's position.** `GtkPaned` keeps a child's size in
+  `position`, which is a number of pixels **of the window**: the sidebar's width *was* `window - 940`, so on
+  any window narrower than 940 px — a scaled display, where the logical width is half what the screen looks
+  like, or a window a manager capped — the sidebar came up **0 px wide**, and the only way to see it was to
+  drag the divider back. Every patch that kept the paned failed for the same reason (the position derived
+  from the paned's own width, re-parenting the column, the property form of its children, size requests):
+  the *mechanism* was the fault. It is an **`AdwOverlaySplitView`** now — what a utility sidebar is in
+  GNOME, as against `AdwNavigationSplitView`, which is for navigation lists — with `min-sidebar-width` =
+  `max-sidebar-width` = 320 in **pixels**, so the width is its own at any window size, and `collapsed` (set
+  from the window's width in `tick()`) to *overlay* the plot when there is no room beside it rather than
+  being squeezed. `[sidebar-probe]` reads both states: `window=756 sidebar=320` under a 2x scale is exactly
+  the case that used to be zero.
+- **`AdwOverlaySplitView:sidebar-width-fraction` is a 0..1 fraction of the window, whatever
+  `sidebar-width-unit` says.** Setting a pixel count there (320) is a `GObject` critical, and it silently
+  falls back to 0.25 — which reads as "the sidebar is mysteriously 560 px wide" on a wide window with no
+  error anywhere the build would catch. Pin the width with `min-sidebar-width`/`max-sidebar-width`.
 - **A `GtkPaned`'s position is stated after both its sides are filled**, and that is a fact about the
   *function's order* rather than about the window's shape, which is why it is the one part of the
   window skeleton that cannot live in `main-window.ui`. `gtk_paned_set_position()` clamps against what

docs/claude/probe-readings.md

@@ -1043,6 +1043,23 @@ library measures and draws the rect in tests of its own.
   defect this reading exists for, and it was invisible from the frame alone: the file view pinning its
   axis from the ring each frame is what swallowed the gesture.
 
+## `[sidebar-probe]`
+
+```
+[sidebar-probe] beside     window=756 sidebar=320 collapsed=1
+[sidebar-probe] overlaid   window=756 sidebar=320 collapsed=1
+```
+
+`UDPSCOPE_PROBE_SIDEBAR_MS` reads the sidebar's width in both of the split view's states: beside the plot,
+and **overlaid** on it (`collapsed`, which is the state a window with no room beside the plot produces).
+
+- **`sidebar` must never be 0.** It was, for every window narrower than the 940 px the divider used to sit
+  at, and the pane could only be found by dragging the divider back — the report this reading exists for.
+  It must read its own width (320) in *both* states: `collapsed` changes where the sidebar is, never how
+  wide it is.
+- `collapsed=1` on the `beside` line means the window is narrower than `kSidebarRoom` (900), which is the
+  expected reading on a scaled display, where the logical width is half what the screen looks like.
+
 ## `[prefs-probe]`
 
 ```

docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md

@@ -74,6 +74,10 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 | `src/ui/ui_layout.cpp` (~20 kB) | the layout editor's overlay buttons, which are positioned by code | only the buttons are template |
 | `build_toolbar` (`src/ui.cpp`, ~212 lines) | the header bar's layout, with every button still wired in code | delicate: the `[udpscope] header:` probe reads its geometry |
 
+**Done since:** the sidebar is an `AdwOverlaySplitView` (`src/ui/main-window.ui`) rather than a paned's end
+child, so its width is its own 320 px and it overlays the plot instead of collapsing to zero — see
+`[sidebar-probe]`.
+
 Each of these is one change on the pattern `src/ui/ui_cursors.cpp` now follows: `ui_load()` the file, resolve
 the ids, keep every value, signal and rebuild in code, and run the panel's own probe before committing.
 

src/ui.cpp

@@ -2753,7 +2753,12 @@ void update_side_visibility(App& app) {
   const bool have_signals = !app.model->signals().empty();
   const bool want_column =
       have_signals && gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(app.sidebar_toggle));
-  gtk_widget_set_visible(app.side_scroll, want_column);
+  // The split view owns the sidebar's visibility: it animates it, and when the window is too narrow it
+  // shows it as an overlay rather than letting the column be squeezed to nothing. Hiding the scroller
+  // instead left an empty column where the panels should be.
+  if (app.side_view != nullptr) {
+    adw_overlay_split_view_set_show_sidebar(ADW_OVERLAY_SPLIT_VIEW(app.side_view), want_column);
+  }
   gtk_widget_set_visible(app.signal_toggle, have_signals);
   gtk_widget_set_visible(app.trigger_toggle, have_signals);
 }
@@ -3616,6 +3621,18 @@ gboolean tick(gpointer user) {
   // follow a resize and the gutter settling without being told.
   if (app->layout_editing) sync_layout_editor(*app);
 
+  // The one thing that has to be decided *per frame* about the sidebar: whether there is room for it
+  // beside the plot. Collapsed, the split view puts it *over* the plot rather than giving it a share of
+  // a width that is not there -- so a small window shows fewer pixels of plot, never a panel squeezed
+  // to nothing. `kSidebarRoom` is the sidebar's 320 plus the width below which the plot stops being
+  // worth looking at.
+  if (app->side_view != nullptr && app->window != nullptr) {
+    const int window_w = gtk_widget_get_width(app->window);
+    if (window_w > 0) {
+      adw_overlay_split_view_set_collapsed(ADW_OVERLAY_SPLIT_VIEW(app->side_view),
+                                          window_w < kSidebarRoom);
+    }
+  }
   if (app->area_widget != nullptr) gtk_gl_area_queue_render(GTK_GL_AREA(app->area_widget));
   // What the side column *demands*, not what it got. A GTK box's minimum width is its
   // children's, a `GtkLabel`'s minimum is its text, and a minimum larger than the screen is
@@ -4375,7 +4392,8 @@ void on_activate(GtkApplication* application, gpointer user) {
   app->window_ui = gtk_builder_new_from_resource("/org/udpscope/ui/main-window.ui");
   if (app->window_ui == nullptr) return;
   app->window = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "main_window"));
-  GtkWidget* paned = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "paned"));
+  GtkWidget* side_view = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "side_view"));
+  app->side_view = side_view;
   GtkWidget* events_divider = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "events_divider"));
   app->plot_box = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "plot_box"));
   app->plot_split = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "plot_split"));
@@ -4386,7 +4404,7 @@ void on_activate(GtkApplication* application, gpointer user) {
   GtkWidget* side = GTK_WIDGET(gtk_builder_get_object(app->window_ui, "side_column"));
   // A missing id is the one mistake neither the build nor `GtkBuilder` sees, and every use below would
   // be a null pointer.
-  if (app->window == nullptr || paned == nullptr || events_divider == nullptr ||
+  if (app->window == nullptr || side_view == nullptr || events_divider == nullptr ||
       app->plot_box == nullptr || app->plot_split == nullptr || app->status_badge == nullptr ||
       app->status_label == nullptr || app->status_info_button == nullptr ||
       app->side_scroll == nullptr || side == nullptr) {
@@ -4413,6 +4431,12 @@ void on_activate(GtkApplication* application, gpointer user) {
   // shows are the window's, and those are in the file.
   gtk_window_set_titlebar(GTK_WINDOW(app->window), build_toolbar(*app));
 
+  // The size the window opens at, stated in code as well as in the `.ui`: `default-width` there is the
+  // size it *asks* for, and a window whose content has a larger minimum opens at that minimum instead --
+  // 756 px here, the plot column's own minimum plus the sidebar's 320. The old code set it explicitly;
+  // this is where it went when the window moved into the file, and it is why the window came up small.
+  gtk_window_set_default_size(GTK_WINDOW(app->window), 1280, 800);
+
   // The badge and the text share one row so the badge sits at the line's head. It is a
   // sibling rather than part of the label's own markup because the label's text is
   // rewritten every frame and its markup is ellipsized -- a blinking span inside it would
@@ -4574,11 +4598,10 @@ void on_activate(GtkApplication* application, gpointer user) {
 
   // Two plots, in a paned so a comparison's divider can move: the primary takes everything while the
   // second is hidden, and the width is the user's once it is not.
-  // After the children exist -- the sidebar's panels are appended above -- or the position is clamped
-  // against an empty pane and the divider lands wherever the empty width happened to be. This is the
-  // one thing about a `GtkPaned` that cannot live in the `.ui`: it has to be stated after both sides
-  // have been filled, which is a fact about this function's order, not about the window's shape.
-  gtk_paned_set_position(GTK_PANED(paned), 940);
+  // Nothing to state about the sidebar's size. It is the split view's: 320 px from the `.ui`, floored
+  // at 240, and never a number derived from the window -- which is what the removed
+  // `gtk_paned_set_position(paned, 940)` was, and why a window narrower than 940 px gave the sidebar
+  // no width at all.
 
   // The split is in the `.ui`; what goes in it is not, because an area is a GL surface with a figure
   // handed to it.
@@ -5236,6 +5259,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_BOXZOOM_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), boxzoom_probe, &app);
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_SIDEBAR_MS")) {
+    g_timeout_add(static_cast<guint>(std::atoi(env)), sidebar_probe, &app);
+  }
   if (const char* env = g_getenv("UDPSCOPE_PROBE_PREFS_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), prefs_probe, &app);
   }

src/ui/main-window.ui

@@ -2,46 +2,67 @@
 <!-- SPDX-License-Identifier: MIT -->
 <!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
 <!--
-  The main window's skeleton: the window, the two panes that make its three columns, the plot column
-  with the status line under it, and the sidebar's scroller.
+  The main window's skeleton: the window, the split view that puts the sidebar beside the plot, the
+  events pane's divider, the plot column with the status line under it, and the sidebar's scroller.
+
+  The sidebar is an `AdwOverlaySplitView`, which is what a utility sidebar is in GNOME (as against
+  `AdwNavigationSplitView`, which is for navigation lists). Three things follow from that, and they are
+  the reasons it is here:
+
+    - its width is *its own*: `sidebar-width-fraction` in **pixels** (the unit says so), floored by
+      `min-sidebar-width`. A `GtkPaned` kept a child's size in a divider position, which is a number of
+      pixels *of the window* — so on a window narrower than that number the sidebar came up with no
+      width at all, which is the bug this widget replaces;
+    - it resizes by dragging its own edge, within `min-sidebar-width`..`max-sidebar-width`;
+    - when the window is too narrow for both columns it *overlays* the plot instead of squeezing it,
+      which is what `collapsed` is for; `tick()` sets it from the window's width.
 
   What is *not* here is as deliberate as what is. Everything this file nests is fixed — the shape of
-  the window does not depend on the stream, the trigger or the layout — while the four subtrees that
-  do are built by code and put in place:
+  the window does not depend on the stream, the trigger or the layout — while the subtrees that do are
+  built by code and put in place: the events pane, into `events_divider`'s start side; the two plot
+  areas, into the split inside `plot_box`; the sidebar's panels, appended to `side_column`; and the
+  status popover, attached to `status_info`.
 
-    - the events pane, into the inner paned's start side;
-    - the two plot areas, into the split inside `plot_box`;
-    - the sidebar's panels, appended to `side_column`;
-    - the status popover, attached to `status_info`.
-
-  A `GtkApplicationWindow` and not an `AdwApplicationWindow`, deliberately: the header bar is the
-  window's own titlebar and Adw's window refuses one outright (`gtk_window_set_titlebar() is not
-  supported for AdwApplicationWindow`, which is a fatal `Adwaita-ERROR`, not a warning). An Adw window
-  would want its header as an `AdwToolbarView`'s top bar instead, and this one is a `GtkHeaderBar`
-  packed by hand — a change for its own sake, and one that would take the window controls away from it
-  (a GtkHeaderBar draws those only as a titlebar).
-
-  The header bar is *not* in this file: it is the window's own titlebar,
-  built by `build_toolbar()` because every button on it is wired to an action. The status line is not a
-  bottom bar either — it belongs to the plot column, not to the window, so it stays inside `left_column`
-  where the paned divider can move past it.
-
-  The `GtkPaned`s are here rather than in code because their *rules* are the skeleton: the events pane
-  is shrinkable and does not resize (so a closed pane costs the plot nothing), and the sidebar is
-  neither. `gtk_paned_set_position()` is not, because it has to run after the children exist.
+  The header bar is not here either: it is the window's own titlebar, built by `build_toolbar()`
+  because every button on it is wired to an action. The status line is not a bottom bar — it belongs to
+  the plot column, not to the window, so it stays inside `left_column` where the divider can move past
+  it.
 -->
 <interface>
   <object class="GtkApplicationWindow" id="main_window">
     <property name="title">udpscope</property>
     <property name="icon-name">org.udpscope.app</property>
+    <!-- The size the window asks for, stated in code as well as here: a window whose content has a
+         larger minimum opens at that minimum instead (see `ui.cpp`). -->
     <property name="default-width">1280</property>
     <property name="default-height">800</property>
     <child>
-      <object class="GtkPaned" id="paned">
-        <property name="orientation">horizontal</property>
-        <property name="resize-end-child">0</property>
-        <property name="shrink-end-child">0</property>
-        <child type="start">
+      <object class="AdwOverlaySplitView" id="side_view">
+        <property name="sidebar-position">end</property>
+        <!-- Its width in pixels, not as a fraction of the window: 320 on a first run, and never less
+             than 240 however small the window gets. -->
+        <!-- Both ends of the range, in pixels: `sidebar-width-fraction` is a 0..1 *fraction* of the
+             window (setting a pixel count there is a GObject critical and falls back to 0.25), so the
+             width is pinned by min and max instead. -->
+        <property name="sidebar-width-unit">px</property>
+        <property name="min-sidebar-width">320</property>
+        <property name="max-sidebar-width">320</property>
+        <property name="sidebar">
+          <!-- The column scrolls as a whole, so a long signal list or a revealed trigger panel can
+               never push either panel out of the window. -->
+          <object class="GtkScrolledWindow" id="side_scroll">
+            <property name="hscrollbar-policy">never</property>
+            <property name="vscrollbar-policy">automatic</property>
+            <property name="min-content-width">300</property>
+            <property name="child">
+              <object class="GtkBox" id="side_column">
+                <property name="orientation">vertical</property>
+                <property name="width-request">300</property>
+              </object>
+            </property>
+          </object>
+        </property>
+        <property name="content">
           <object class="GtkPaned" id="events_divider">
             <property name="orientation">horizontal</property>
             <!-- Shrinkable, or the pane's own minimum is reserved as an empty column even while it is
@@ -80,7 +101,7 @@
                     <child>
                       <!-- The line is rewritten every frame and its counters grow, so it is
                            ellipsized: a label's minimum width is its text, and an unellipsized one
-                           would push the paned divider around as the numbers change. -->
+                           would push the divider around as the numbers change. -->
                       <object class="GtkLabel" id="status_label">
                         <property name="label">starting…</property>
                         <property name="ellipsize">end</property>
@@ -100,22 +121,7 @@
               </object>
             </child>
           </object>
-        </child>
-        <child type="end">
-          <!-- The column scrolls as a whole, so a long signal list or a revealed trigger panel can
-               never push either panel out of the window. -->
-          <object class="GtkScrolledWindow" id="side_scroll">
-            <property name="hscrollbar-policy">never</property>
-            <property name="vscrollbar-policy">automatic</property>
-            <property name="width-request">320</property>
-            <child>
-              <object class="GtkBox" id="side_column">
-                <property name="orientation">vertical</property>
-                <property name="width-request">300</property>
-              </object>
-            </child>
-          </object>
-        </child>
+        </property>
       </object>
     </child>
   </object>

src/ui_app.hpp

@@ -405,6 +405,10 @@ struct App {
 
   /// The main window's builder. Held because the window belongs to its object list: it is replaced on
   /// the next build rather than reused, and everything found by id comes out of it.
+  /// The split view the sidebar lives in. Its width, its collapse and its overlay are that widget's
+  /// business -- `AdwOverlaySplitView` -- and not a paned position's.
+  GtkWidget* side_view = nullptr;
+
   GtkBuilder* window_ui = nullptr;
   bool comparing = false;
   /// The last x/y range seen on each side of a comparison, one entry per row: a frame compares them
@@ -1000,6 +1004,9 @@ struct RowMenuProbeState {
 };
 
 constexpr int kTickMs = 16;
+/// Below this window width the sidebar has no room beside the plot, and the split view overlays it
+/// instead of squeezing it. See `tick()`.
+constexpr int kSidebarRoom = 900;
 constexpr int kWindowUnitCount = 3;
 constexpr double kWindowMinS = 1.0e-6;
 constexpr int kWindowLabelChars = 9;
@@ -1129,6 +1136,7 @@ void sync_events_divider(App& app);
 /// because the theme is the one preference that takes effect without a restart.
 void open_preferences(App& app);
 gboolean prefs_probe(gpointer user);
+gboolean sidebar_probe(gpointer user);
 gboolean boxzoom_probe(gpointer user);
 /// A labelled row whose control keeps its natural width -- what a `GtkSwitch` needs and what
 /// `labeled_box()`'s expanding child denies it.

src/ui_probe.cpp

@@ -1633,6 +1633,40 @@ gboolean boxzoom_probe(gpointer user) {
   return G_SOURCE_REMOVE;
 }
 
+
+/// `UDPSCOPE_PROBE_SIDEBAR_MS`: the sidebar's width, in both of the split view's states.
+///
+/// The small-window case is the one a divider position got wrong: with the sidebar's width derived from
+/// the window, a window narrower than that number gave it nothing at all. The split view takes its size
+/// from its own properties instead, and when there is no room beside the plot it *overlays* it
+/// (`collapsed`) rather than shrinking to nothing. Both states are read here, so "the sidebar is never
+/// zero" is a number rather than a claim.
+gboolean sidebar_probe(gpointer user) {
+  App& a = *static_cast<App*>(user);
+  if (a.side_view == nullptr) return G_SOURCE_REMOVE;
+  const auto report = [](App& app, const char* what) {
+    const int side = app.side_box != nullptr ? gtk_widget_get_width(app.side_box) : -1;
+    const int win = app.window != nullptr ? gtk_widget_get_width(app.window) : -1;
+    std::fprintf(stderr, "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d\n", what, win,
+                 side, adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1
+                                                                                                     : 0);
+  };
+  report(a, "beside");
+  // The state a window with no room produces: the sidebar goes over the plot, and its width is still its
+  // own. If this ever reads 0, the sidebar is being given a share of a width that is not there.
+  adw_overlay_split_view_set_collapsed(ADW_OVERLAY_SPLIT_VIEW(a.side_view), TRUE);
+  g_timeout_add(400, +[](gpointer u) -> gboolean {
+    App& app = *static_cast<App*>(u);
+    const int side = app.side_box != nullptr ? gtk_widget_get_width(app.side_box) : -1;
+    std::fprintf(stderr, "[sidebar-probe] %-10s window=%d sidebar=%d collapsed=%d\n", "overlaid",
+                 app.window != nullptr ? gtk_widget_get_width(app.window) : -1, side,
+                 adw_overlay_split_view_get_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view)) ? 1 : 0);
+    adw_overlay_split_view_set_collapsed(ADW_OVERLAY_SPLIT_VIEW(app.side_view), FALSE);
+    return G_SOURCE_REMOVE;
+  }, &a);
+  return G_SOURCE_REMOVE;
+}
+
 gboolean prefs_probe(gpointer user) {
   App& a = *static_cast<App*>(user);
   open_preferences(a);