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

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

src/ui_app.hpp

@@ -209,7 +209,8 @@ struct App {
   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.
+  /// 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;
@@ -1622,6 +1623,11 @@ constexpr const char* kLayoutCellKey = "udpscope-layout-cell";
 constexpr const char* kLayoutColKey = "udpscope-layout-col";
 /// The minimum width a cell may be dragged to, in logical pixels.
 constexpr double kMinCellPx = 96.0;
+/// The plotter's own per-cell floor, `kMinCellWidthLogical` in `deps/plotter/src/plot/figure.cpp`
+/// (`place_cells()`): a cell is `floor + extra * weight / total`. Both it and `kMinCellPx` are
+/// *logical* px -- the plotter scales its floor by the `ui_scale` the renderer installs, which is
+/// `PlotArea::device_scale()`, the same divisor that turns a layout rect into logical px here.
+constexpr double kPlotterMinCellPx = 64.0;
 constexpr gint64 kMeasureRefreshUs = 100 * 1000;
 constexpr gint64 kMeasureHeavyUs = 5 * 1000;
 constexpr const char* kCursorModeNames[] = {"vertical", "horizontal", "both", "mix"};
@@ -2110,6 +2116,9 @@ gboolean window_probe(gpointer user);
 GtkWidget* layout_button(App& a, bool insert, int index);
 GtkWidget* layout_cell_button(App& a, int fr, int pos);
 GtkWidget* layout_col_handle(App& a, int plot);
+/// Records the starting widths (logical px) and weights of the cell pair a column drag on `plot`
+/// (the right-hand cell) begins with. False when `plot` has no left neighbour.
+bool layout_col_drag_begin(App& app, int plot);
 void layout_col_apply(App& app, int plot, double offset_px);
 void layout_probe_report(App& a, const char* what);
 /// The split in the report lines, from the `RowSet` alone (no widget is asked, so the exit dump can