fix(plot): a figure gives every row the same y gutter so the axes line up

include/plot/plot/figure.hpp

@@ -136,6 +136,14 @@ class Figure {
   /// This is also where a row's `PlotLayout` becomes well-defined: a row's chrome is
   /// laid out against `rows[i]` by its own `Plot::layout()`, which is why the figure
   /// computes rects and not axes.
+  ///
+  /// It is also the one place the rows' left chrome can be made to agree (spec §16):
+  /// each row is laid out once to learn the gutter its own labels and title column
+  /// need, the figure takes the maximum and imposes it on every row through
+  /// `Plot::set_min_y_gutter()` before laying the rows out once more -- so every row's
+  /// `axes.x` and `axes.w` are equal and a vertical feature lands in the same pixel
+  /// column in each. Two passes per row per call, not a loop: the gutter is a maximum
+  /// over rows and a row's own strip width does not depend on it.
   const FigureLayout& layout();
   const FigureLayout& layout() const noexcept { return layout_; }
 

include/plot/plot/plot.hpp

@@ -524,6 +524,24 @@ class Plot {
   /// a lookup on a const Plot cannot move the axes out from under the caller.
   const PlotLayout& layout() const noexcept { return layout_; }
 
+  /// A floor on the left gutter `layout()` reserves, in device pixels: the label
+  /// strip and the title column together are never narrower than this, so `axes`
+  /// never starts left of `viewport().x + min_y_gutter()`. 0.0 -- the default --
+  /// imposes nothing and the plot sizes its strip from its own widest tick label,
+  /// which is what a plot standing on its own has always done.
+  ///
+  /// It exists for `Figure`, which is the only thing that can know the answer: a row
+  /// cannot see its siblings' tick labels, so the figure lays each row out once to
+  /// learn the gutter it needs, takes the maximum and sets it here on every row --
+  /// after which all the rows' `axes.x` and `axes.w` agree and a vertical feature
+  /// lands in the same pixel column in each. The floor is applied inside the same cap
+  /// as the measured strip, so a figure's gutter can never squeeze `axes` below the
+  /// half-viewport floor either. Negative values are clamped to 0.
+  void set_min_y_gutter(f32 device_px) noexcept {
+    min_y_gutter_ = device_px > 0.0f ? device_px : 0.0f;
+  }
+  f32 min_y_gutter() const noexcept { return min_y_gutter_; }
+
   /// Grid segments for the primary axis pair (x axis 0, y axis 0), in data space.
   const Grid& grid() const noexcept { return grid_; }
 
@@ -713,6 +731,9 @@ class Plot {
   Legend legend_{};
   /// Which parts of the bottom x chrome are reserved at all; see set_x_strip_mode().
   XStrip x_strip_ = XStrip::Both;
+  /// The floor a figure imposes on the left gutter; see set_min_y_gutter(). 0.0 means
+  /// none, which is what a plot standing on its own keeps.
+  f32 min_y_gutter_ = 0.0f;
   /// Which handles a pointer is on, published by the input layer (see HotHandles).
   HotHandles hot_{};
 

src/plot/figure.cpp

@@ -393,12 +393,41 @@ const FigureLayout& Figure::layout() {
   // its transforms describe this frame's rects; and before the rows lay themselves
   // out below, so a following group's window is the range they lay out with.
   sync_x_links();
-  // Every row is laid out against its own rect here, so after this call each row's
+  // Every row is laid out against its own rect, so after this call each row's
   // chrome is current: `plot(i).layout()` reads back what the figure computed rather
   // than what the row last held. That is the contract `Renderer::draw(Figure&)`
   // relies on -- it draws each row with the layout this produced, and `draw(Plot&)`
   // is the one that computes its own.
-  for (u32 row = 0; row < rows; ++row) plots_[row].layout();
+  //
+  // The rows' axes have to line up (spec §16), so this is where the figure's **common
+  // left gutter** is decided -- a row cannot see its siblings' tick labels, so the one
+  // width they can agree on has to be handed to them from here.
+  //
+  // Two passes, and no more. The first lays every row out with no floor, so each
+  // reports the gutter its own labels and title column need; the figure keeps the
+  // maximum. The second re-lays every row with that maximum imposed on its
+  // `Plot::set_min_y_gutter()`, which widens a row's strip to fill it, so all the rows'
+  // `axes` start and end on the same columns. A fixed-point loop -- re-measuring until
+  // the rows stop moving -- is the tempting wrong answer and is not needed: the gutter
+  // is a *maximum* over rows, and a row's own strip width does not depend on any other
+  // row's or on the width imposed on it (y tick labels are chosen from the row's own
+  // range and height, and a title column is one text line per titled axis), so one read
+  // per row settles the answer exactly. Measuring with a floor from the previous frame
+  // would be the only reason to iterate, and clearing it before the measurement pass is
+  // what removes it.
+  f32 gutter = 0.0f;
+  for (u32 row = 0; row < rows; ++row) {
+    plots_[row].set_min_y_gutter(0.0f);
+    const PlotLayout& row_layout = plots_[row].layout();
+    gutter = std::max(gutter, row_layout.y_axis_width + row_layout.y_title_width);
+  }
+  // A one-row figure imposes exactly what that row measured, so it lays out as the plot
+  // would on its own; a figure whose rows all need the same strip imposes that, which is
+  // also what each would have done alone.
+  for (u32 row = 0; row < rows; ++row) {
+    plots_[row].set_min_y_gutter(gutter);
+    plots_[row].layout();
+  }
 
   // The figure's one x axis is the bottom row's, so the strip it is drawn in is that
   // row's own label area -- the rect the controller hit-tests and the renderer stages

src/plot/plot.cpp

@@ -836,7 +836,14 @@ const PlotLayout& Plot::layout() {
     x_height = std::clamp(x_height, 0.0f, viewport_.h);
     const f32 y_budget = (1.0f - kMinAxesShare) * viewport_.w;
     const f32 x_budget = (1.0f - kMinAxesShare) * viewport_.h;
-    y_width = std::min(y_width, y_budget);
+    // The figure's common gutter, when one was imposed: a floor on `y_width + y_title_w`
+    // so every row's axes starts at the same x. It is served by the *label* strip -- the
+    // title column is one text line per titled axis and cannot be widened to fill a gutter
+    // -- and it is applied before the title column below, so the pair still fits the cap
+    // (the floor is clamped to the same budget) and the invariant `axes.w >= share` holds.
+    // `min_y_gutter_` is 0.0 unless a figure set it, so this is a no-op for a lone plot.
+    const f32 y_floor = std::min(min_y_gutter_, y_budget);
+    y_width = std::min(std::max(y_width, y_floor - y_title_w), y_budget);
     x_height = std::min(x_height, x_budget);
     const f32 y_title_width = std::min(y_title_w, y_budget - y_width);
     const f32 x_title_height = std::min(x_title_h, x_budget - x_height);
@@ -865,8 +872,14 @@ const PlotLayout& Plot::layout() {
     const f32 need_x =
         x_labels ? std::min(std::clamp(text_->height() + 2.0f * pad, 0.0f, viewport_.h), x_budget)
                  : 0.0f;
-    if (need_y == y_width && need_x == x_height) break;  // strips are a fixed point
-    y_width = need_y;
+    // What the strip *wants* is the label's own width or the figure's floor, whichever is
+    // greater -- the floor is a maximum over sources, settled by one `max` and not by
+    // asking the loop to grow the strip towards it pass by pass. Comparing the measured
+    // need alone would never agree once a floor sits above it, and the loop would run its
+    // cap out; comparing the want is what keeps it a fixed point here too.
+    const f32 want_y = std::min(std::max(need_y, y_floor - y_title_w), y_budget);
+    if (want_y == y_width && need_x == x_height) break;  // strips are a fixed point
+    y_width = want_y;
     x_height = need_x;
   }
 

tests/test_figure.cpp

@@ -43,6 +43,42 @@ std::vector<plot::Rect> rows_of(plot::Figure& figure) {
   return figure.layout().rows;
 }
 
+/// Gives one plot the axes of the misalignment cases: a fixed x range and a fixed y
+/// range from `give_magnitude_row()`'s table. Fixed (`auto_scale` off), so the labels a row
+/// measures are the ones its magnitude produces whatever order the rows are laid out
+/// in -- which is what makes two rows' strips comparable.
+void give_magnitude_row(plot::Plot& p, plot::u32 which) {
+  static plot::render::EmbeddedMonoFont font;
+  const plot::Range y_ranges[3] = {{0.0, 671.0e6}, {0.0, 90.0}, {0.0, 2.51}};
+  p.set_text_backend(&font);
+  p.add_x_axis({"t", {0.0, 100.0}, false, plot::AxisPlace::Bottom, false});
+  p.add_y_axis({"v", y_ranges[which % 3u], false, plot::AxisPlace::Left, false});
+}
+
+/// A figure whose rows carry deliberately different magnitudes: the "671m"-scale
+/// row's tick labels are four characters wide, the "90" row's two and the "2.51"
+/// row's three. That is the shape the reported misalignment had -- each row sized its
+/// own y strip from its own widest label, so the rows' axes started at three different
+/// x.
+plot::Figure different_magnitude_figure(plot::u32 rows = 3u) {
+  plot::Figure figure(rows);
+  figure.set_viewport(whole_figure());
+  for (plot::u32 row = 0; row < rows; ++row) give_magnitude_row(figure.plot(row), row);
+  return figure;
+}
+
+/// The left gutter `which`'s row would reserve for itself if it were a plot *standing
+/// alone* in `viewport` -- the label strip plus the title column, which is the width a
+/// figure's common gutter is a maximum over. The same axes, text backend and metrics,
+/// laid out with no figure floor, so reading it leaves the figure's rows untouched.
+plot::f32 natural_gutter_width(plot::u32 which, plot::Rect viewport) {
+  plot::Plot solo;
+  give_magnitude_row(solo, which);
+  solo.set_viewport(viewport);
+  solo.layout();
+  return solo.layout().y_axis_width + solo.layout().y_title_width;
+}
+
 bool rows_overlap(const plot::Rect& a, const plot::Rect& b) {
   return a.x < b.right() && b.x < a.right() && a.y < b.bottom() && b.y < a.bottom();
 }
@@ -255,6 +291,135 @@ PLOT_TEST(every_row_keeps_a_positive_height) {
   }
 }
 
+// ------------------------------------------------------- y gutter alignment
+
+/// Spec §16, the invariant: a figure gives its rows one common left gutter, so every
+/// row's `axes` starts and ends on the same pixel columns. This is the mechanism; the
+/// case below is the drawn consequence a user sees as "the y axes line up".
+PLOT_TEST(figure_rows_share_one_y_gutter) {
+  plot::Figure figure = different_magnitude_figure();
+  const plot::FigureLayout layout = figure.layout();
+  const plot::Rect first = figure.plot(0u).layout().axes;
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::Rect& axes = figure.plot(row).layout().axes;
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.x), static_cast<plot::f64>(first.x), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.w), static_cast<plot::f64>(first.w), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.right()), static_cast<plot::f64>(first.right()),
+                    1e-4);
+    // The rows are still their own plots vertically: the equality is the *left gutter*
+    // and not three rows accidentally given one rect.
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.y), static_cast<plot::f64>(layout.rows[row].y),
+                    1e-4);
+  }
+  // The rows really do disagree about the strip they need, which is what makes the
+  // gutter a decision the figure has to make: each row's own labels, measured with no
+  // floor in the row's own viewport, ask for a different width -- and every row's
+  // applied width is at least what it asked for, so the figure widened the narrow ones
+  // rather than inventing a width they did not need.
+  plot::f32 natural[3] = {};
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    natural[row] = natural_gutter_width(row, layout.rows[row]);
+    const plot::PlotLayout& rl = figure.plot(row).layout();
+    PLOT_CHECK(rl.y_axis_width + rl.y_title_width >= natural[row] - 1e-4f);
+  }
+  PLOT_CHECK(!(natural[0] == natural[1] && natural[1] == natural[2]));
+  // The shared x axis is still the *bottom row's own* label area -- aligning the rows'
+  // left chrome must not move where a press on the shared axis lands.
+  const plot::PlotLayout& bottom = figure.plot(2u).layout();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.x),
+                  static_cast<plot::f64>(bottom.x_label_area.x), 1e-4);
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.w),
+                  static_cast<plot::f64>(bottom.x_label_area.w), 1e-4);
+  PLOT_CHECK(layout.x_axis_strip.h > 0.0f);
+}
+
+/// Spec §16, the drawn consequence: with one gutter, a given data x maps to **one**
+/// pixel column of the figure, because every row's x transform is built from the same
+/// axes rect. That is the property a vertical feature -- a cursor, a stacked sample --
+/// relies on, and it is what "the y axis are not vertically aligned" complained about.
+PLOT_TEST(figure_rows_map_a_data_x_to_one_column) {
+  plot::Figure figure = different_magnitude_figure();
+  figure.layout();
+  const plot::f32 column = static_cast<plot::f32>(figure.plot(0u).x_transform(0u).to_screen_x(37.5));
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::Plot& p = figure.plot(row);
+    const plot::f32 x = static_cast<plot::f32>(p.x_transform(0u).to_screen_x(37.5));
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(x), static_cast<plot::f64>(column), 1e-3);
+    // ... and the leftmost data column lands on the axes' left edge in every row, so
+    // the alignment is the full width and not a common offset into three different ones.
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(p.x_transform(0u).to_screen_x(0.0)),
+                    static_cast<plot::f64>(p.layout().axes.x), 1e-3);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(p.x_transform(0u).to_screen_x(100.0)),
+                    static_cast<plot::f64>(p.layout().axes.right()), 1e-2);
+  }
+}
+
+/// The neighbour: a `Plot` standing alone is not a figure and keeps sizing its strip
+/// from its own data. The floor is the figure's imposition, unset by default, and the
+/// only thing that widens a lone plot's *gutter* is an explicit one.
+PLOT_TEST(a_plot_on_its_own_keeps_its_own_y_gutter) {
+  plot::Plot p;
+  give_magnitude_row(p, 1u);  // the "90" row: the narrowest labels
+  p.set_viewport(whole_figure());
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(p.min_y_gutter()), 0.0, 1e-12);
+  const plot::PlotLayout& natural_layout = p.layout();
+  const plot::f32 natural = natural_layout.y_axis_width + natural_layout.y_title_width;
+  PLOT_CHECK(natural > 0.0f);
+
+  // An explicit floor widens the gutter, which is the same lever `Figure` pulls -- and
+  // it is a floor on the *gutter*, so the label strip and the title column together
+  // reach it rather than the label strip alone.
+  p.set_min_y_gutter(natural + 20.0f);
+  const plot::PlotLayout& wide = p.layout();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(wide.y_axis_width + wide.y_title_width),
+                  static_cast<plot::f64>(natural + 20.0f), 1e-3);
+  // Negative values mean "no floor", like `set_row_gap()`, and the gutter goes back to
+  // what the labels and titles ask for.
+  p.set_min_y_gutter(-5.0f);
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(p.min_y_gutter()), 0.0, 1e-12);
+  const plot::PlotLayout& back = p.layout();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(back.y_axis_width + back.y_title_width),
+                  static_cast<plot::f64>(natural), 1e-4);
+}
+
+/// The figure's gutter is clamped by the same cap as the measured strip, so a figure
+/// whose rows are much wider than its viewport still cannot let the gutter squeeze the
+/// axes below the half-viewport floor `Plot::layout()` guarantees.
+PLOT_TEST(figure_gutter_never_squeezes_the_axes_below_the_floor) {
+  plot::Figure figure = different_magnitude_figure();
+  const plot::f32 w = 40.0f;
+  figure.set_viewport({0.0f, 0.0f, w, 60.0f});
+  figure.layout();
+  const plot::Rect first = figure.plot(0u).layout().axes;
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::Rect& axes = figure.plot(row).layout().axes;
+    PLOT_CHECK(axes.w >= 0.5f * w - 1e-3f);
+    PLOT_CHECK(axes.w > 0.0f);
+    // ... and the rows still agree, which is the point: the cap is the figure's width,
+    // and every row shares it.
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.x), static_cast<plot::f64>(first.x), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.w), static_cast<plot::f64>(first.w), 1e-4);
+  }
+}
+
+/// The degenerate neighbour: a figure of one row imposes exactly what that row
+/// measured, so it lays out as the plot would on its own -- the gutter cannot move a
+/// row that has no sibling to line up with.
+PLOT_TEST(a_one_row_figure_lays_out_as_the_plot_alone_does) {
+  plot::Figure figure = different_magnitude_figure(1u);
+  const plot::FigureLayout layout = figure.layout();
+  plot::Plot solo;
+  give_magnitude_row(solo, 0u);
+  solo.set_viewport(layout.rows[0]);
+  solo.layout();
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.plot(0u).layout().axes.x),
+                  static_cast<plot::f64>(solo.layout().axes.x), 1e-4);
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.plot(0u).layout().axes.w),
+                  static_cast<plot::f64>(solo.layout().axes.w), 1e-4);
+  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.plot(0u).layout().y_axis_width),
+                  static_cast<plot::f64>(solo.layout().y_axis_width), 1e-4);
+}
+
 // --------------------------------------------------------------- the x link
 
 /// Linking a row to a group that already has a range puts it on that range at once,

tests/test_render_smoke.cpp

@@ -3677,6 +3677,113 @@ PLOT_TEST(shared_x_axis_is_drawn_once) {
   ctx.destroy();
 }
 
+/// Spec §16, end to end: a figure whose rows carry different magnitudes ("671m",
+/// "90", "2.51" scale tick labels) draws a vertical feature -- each row's own cursor
+/// at the same data x -- in the **same pixel column** in every row. That is what a
+/// user sees when the y axes line up, and it is what the rows' `axes` rects being
+/// equal is for: the equality below is read from the drawn frame, not from the layout
+/// alone, so a renderer that derived a row's geometry of its own would fail here.
+PLOT_TEST(figure_rows_line_up_a_vertical_cursor) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+  plot::Renderer r;
+  PLOT_CHECK(r.init(&st));
+
+  const std::vector<plot::f32> samples = wave(101u, 0.0f);
+  plot::data::ArraySeries series(samples, 0.0, 1.0);
+
+  // The cursor's colour, and the only strongly red thing in these frames: the trace is
+  // `kRowBlue` and every piece of chrome is grey (all three channels equal), so a
+  // red-dominant pixel inside a row's `axes` can only be that row's cursor. The row
+  // ranges are the three magnitudes whose tick labels are four, two and three
+  // characters wide -- the shape that gave each row its own strip before the gutter.
+  const plot::Color cursor_red = plot::Color::hex(0xFF2020);
+  const plot::f32 y_scales[3] = {671.0e6f, 90.0f, 2.51f};
+
+  plot::Figure figure(3u);
+  figure.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    plot::Plot& p = figure.plot(row);
+    const plot::u32 ax = p.add_x_axis({"t", {0.0, 100.0}, false, plot::AxisPlace::Bottom, false});
+    const plot::u32 ay =
+        p.add_y_axis({"v", {0.0, y_scales[row]}, false, plot::AxisPlace::Left, false});
+    p.add_trace({&series, kRowBlue, 1.5f, ax, ay});
+    p.add_cursor(50.0, cursor_red);
+  }
+
+  r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  r.draw(figure);
+  r.end_frame();
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+  dump_if_requested(r, "figure_gutter_cursor.ppm");
+
+  const std::vector<plot::u8> frame = read_frame(ctx);
+  const plot::FigureLayout layout = figure.layout();
+  PLOT_CHECK(layout.rows.size() == 3u);
+  // The mechanism, straight from the drawn frame's own layouts: every row's axes starts
+  // and ends on the same pixel columns, and that is true at the top of the figure too --
+  // these rows are separate plots vertically and one column horizontally.
+  const plot::Rect first_axes = figure.plot(0u).layout().axes;
+  for (plot::u32 row = 1; row < 3u; ++row) {
+    const plot::Rect& axes = figure.plot(row).layout().axes;
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.x), static_cast<plot::f64>(first_axes.x), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.w), static_cast<plot::f64>(first_axes.w), 1e-4);
+    PLOT_CHECK_NEAR(static_cast<plot::f64>(axes.right()),
+                    static_cast<plot::f64>(first_axes.right()), 1e-4);
+  }
+
+  // Every column of `area` that holds a *fully covered* cursor pixel, left to right.
+  // The predicate demands the cursor red at full coverage (0xFF2020), which is what
+  // makes the comparison across rows meaningful: a partially covered column along the
+  // stroke's edge blends with whatever the row has under it -- its trace at a different
+  // y -- so its colour is a property of that row's data, not of the geometry this case
+  // is about. The core of the stroke has no such dependence.
+  const auto cursor_columns = [&](const plot::Rect& area) {
+    std::vector<plot::u32> columns;
+    for (plot::u32 x = static_cast<plot::u32>(area.x);
+         x < static_cast<plot::u32>(area.right()); ++x) {
+      for (plot::u32 y = static_cast<plot::u32>(area.y);
+           y < static_cast<plot::u32>(area.bottom()); ++y) {
+        const plot::u8* px = &frame[(plot::usize(y) * ctx.width + x) * 4u];
+        if (px[0] >= 200 && px[1] <= 80 && px[2] <= 80) {
+          columns.push_back(x);
+          break;
+        }
+      }
+    }
+    return columns;
+  };
+
+  std::vector<plot::u32> first_columns;
+  for (plot::u32 row = 0; row < 3u; ++row) {
+    const plot::PlotLayout& row_layout =
+        static_cast<const plot::Plot&>(figure.plot(row)).layout();
+    const std::vector<plot::u32> columns = cursor_columns(row_layout.axes);
+    PLOT_CHECK(!columns.empty());  // the row's cursor really is in the frame
+    // One connected run: the stroke is one vertical line, not a doubled or smeared one.
+    PLOT_CHECK(columns.back() - columns.front() + 1u == columns.size());
+    if (row == 0) {
+      first_columns = columns;
+    } else {
+      // The same pixel column in this row as in the first -- the drawn consequence of
+      // the shared gutter, which a frame that sized each row's strip from its own
+      // labels cannot satisfy.
+      PLOT_CHECK(columns == first_columns);
+    }
+  }
+  // ... and that column is the one the shared transform predicts, so the equality is
+  // the geometry's and not three cursors that happened to land together. The tolerance
+  // is the stroke's own pixel: the transform gives the stroke's centre, which lands
+  // inside the band it inked.
+  const plot::u32 expected = static_cast<plot::u32>(
+      static_cast<const plot::Plot&>(figure.plot(0u)).x_transform(0u).to_screen_x(50.0));
+  PLOT_CHECK(first_columns.front() <= expected + 1u && expected <= first_columns.back() + 1u);
+
+  ctx.destroy();
+}
+
 /// A resize recomputes the layout: the rows are re-derived from the new viewport --
 /// their rects, the shared strip, the axes and the transforms they pin -- rather than
 /// rescaled from, or left at, the rects of the frame before. The second frame here is