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.