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

src/ui_app.hpp

@@ -208,9 +208,10 @@ struct App {
   /// began, so the beginning has to be kept -- reading the weights while dragging would compound.
   int layout_drag_row = -1;
   std::vector<float> layout_drag_start;
-  /// Column drag state: which plot (right-hand cell), the two starting widths in logical px,
-  /// and the cell weights snapshot at drag begin (`layout_col_apply()` redistributes the pair's
-  /// weight sum from these, so a drag stays relative to where it started).
+  /// Column drag state: which plot (right-hand cell), the two starting *plot* widths in logical px
+  /// (a cell's rect minus the y gutter only the first cell of a row has), and the cell weights
+  /// snapshot at drag begin (`layout_col_apply()` redistributes the pair's weight sum from these, so
+  /// a drag stays relative to where it started).
   int layout_col_drag_plot = -1;
   double layout_col_drag_left0 = 0.0;
   double layout_col_drag_right0 = 0.0;