layout editor: column drag measures plot widths, not cell rects

src/ui.cpp

@@ -753,6 +753,8 @@ void on_layout_drag_end(GtkGestureDrag*, double, double, gpointer user) {
 
 // --- column (cell) resize drag -----------------------------------------------
 
+// The two cells' widths here are *plot* widths (a cell's rect without the first cell's y gutter): the
+// 96 px floor and the plotter's 64 px floor both apply to them, and the weights share them out.
 void layout_col_apply(App& app, int plot, double offset_px) {
   RowSet& rs = *app.rows;
   const int fr = rs.figure_row_of(plot);
@@ -792,8 +794,18 @@ bool layout_col_drag_begin(App& app, int plot) {
   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;
-  app.layout_col_drag_left0 = static_cast<double>(fl.cells[static_cast<std::size_t>(plot - 1)].w) / scale;
-  app.layout_col_drag_right0 = static_cast<double>(fl.cells[static_cast<std::size_t>(plot)].w) / scale;
+  // The weights share out *plot* widths, so the pair is measured as plots: a cell's rect minus the y
+  // gutter (axis strip + title column) that only the first cell of a row has. The left cell of the pair
+  // is the first one when `plot - 1` starts the row; the right cell never has a gutter, and neither does
+  // a non-first left cell (its plot's strips read 0).
+  const auto plot_width = [&](int cell) {
+    const RowSet::Row& r = app.rows->row(cell);
+    const double gutter =
+        r.plot != nullptr ? double(r.plot->layout().y_axis_width + r.plot->layout().y_title_width) : 0.0;
+    return std::max(0.0, double(fl.cells[static_cast<std::size_t>(cell)].w) - gutter);
+  };
+  app.layout_col_drag_left0 = plot_width(plot - 1) / scale;
+  app.layout_col_drag_right0 = plot_width(plot) / 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;