layout editor: column drag follows the pointer through the cell floor

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ui.cpp

@@ -758,30 +758,37 @@ void layout_col_apply(App& app, int plot, double offset_px) {
   const int fr = rs.figure_row_of(plot);
   const int k = rs.cell_of(plot);
   if (fr < 0 || k < 1) return;
+  // The weights the drag began with: a drag is relative to its start, so reading the live weights here
+  // would compound with every update.
+  if (app.layout_col_drag_start.size() != static_cast<std::size_t>(rs.cells_in_row(fr))) return;
   const double scale =
       app.area != nullptr && app.area->device_scale() > 0.0f ? app.area->device_scale() : 1.0;
   const double pair = app.layout_col_drag_left0 + app.layout_col_drag_right0;
   const double floor = std::min(kMinCellPx, pair * 0.5);
   const double want_left = std::clamp(app.layout_col_drag_left0 + offset_px / scale,
                                       floor, pair - floor);
-  std::vector<float> w = rs.cell_weights(fr);
-  w.resize(static_cast<std::size_t>(rs.cells_in_row(fr)), 1.0f);
+  // The plotter lays a cell out as `floor + extra * weight / total`, so a width is *not* proportional to
+  // its weight: the pair's `pair` px are `2 * floor` plus `(pair - 2 * floor)` shared by the two weights.
+  // Solve for the share that puts the left cell at `want_left`; the pair's weight sum stays what it was,
+  // which keeps the pair's total width (and every other cell) where it was.
+  const double spare = pair - 2.0 * kPlotterMinCellPx;
+  const double frac = spare > 0.0
+                          ? std::clamp((want_left - kPlotterMinCellPx) / spare, 0.001, 0.999)
+                          : 0.5;
+  std::vector<float> w = app.layout_col_drag_start;
   const float sum = w[static_cast<std::size_t>(k - 1)] + w[static_cast<std::size_t>(k)];
-  w[static_cast<std::size_t>(k - 1)] = static_cast<float>(sum * want_left / pair);
+  w[static_cast<std::size_t>(k - 1)] = static_cast<float>(sum * frac);
   w[static_cast<std::size_t>(k)]     = sum - w[static_cast<std::size_t>(k - 1)];
   rs.set_cell_weights(fr, w);
   if (app.area_widget != nullptr) gtk_gl_area_queue_render(GTK_GL_AREA(app.area_widget));
 }
 
-void on_layout_col_begin(GtkGestureDrag* gesture, double, double, gpointer user) {
-  App& app = *static_cast<App*>(user);
-  app.layout_col_drag_plot =
-      GPOINTER_TO_INT(g_object_get_data(G_OBJECT(gesture), "udpscope-layout-col-drag"));
-  const int plot = app.layout_col_drag_plot;
-  if (plot < 0 || plot >= app.rows->row_count()) return;
+bool layout_col_drag_begin(App& app, int plot) {
+  app.layout_col_drag_plot = plot;
+  if (app.rows == nullptr || plot < 1 || plot >= app.rows->row_count()) return false;
   const int fr = app.rows->figure_row_of(plot);
   const int k = app.rows->cell_of(plot);
-  if (fr < 0 || k < 1) return;
+  if (fr < 0 || k < 1) return false;
   const plot::FigureLayout& fl = app.rows->figure().layout();
   const double scale =
       app.area != nullptr && app.area->device_scale() > 0.0f ? app.area->device_scale() : 1.0;
@@ -789,6 +796,13 @@ void on_layout_col_begin(GtkGestureDrag* gesture, double, double, gpointer user)
   app.layout_col_drag_right0 = static_cast<double>(fl.cells[static_cast<std::size_t>(plot)].w) / scale;
   app.layout_col_drag_start = app.rows->cell_weights(fr);
   app.layout_col_drag_start.resize(static_cast<std::size_t>(app.rows->cells_in_row(fr)), 1.0f);
+  return true;
+}
+
+void on_layout_col_begin(GtkGestureDrag* gesture, double, double, gpointer user) {
+  App& app = *static_cast<App*>(user);
+  layout_col_drag_begin(
+      app, GPOINTER_TO_INT(g_object_get_data(G_OBJECT(gesture), "udpscope-layout-col-drag")));
 }
 
 void on_layout_col_update(GtkGestureDrag*, double offset_x, double, gpointer user) {