adapters: nearest-cell distance in double

adapters/gtk4/gtk_plot_area.cpp

@@ -326,16 +326,17 @@ u32 PlotArea::row_in(const std::vector<Rect>& rows, f32 x, f32 y) noexcept {
   // in a visible gap must not default to plot 0. For a figure with one cell per row
   // every cell spans the figure's width, so this reduces to the nearest row by y.
   u32 row = 0;
-  f32 best = std::numeric_limits<f32>::infinity();
+  double best = std::numeric_limits<double>::infinity();
   for (u32 i = 0; i < rows.size(); ++i) {
     const Rect& r = rows[i];
-    const f32 dx = x < r.x ? r.x - x : (x >= r.right() ? x - r.right() : 0.0f);
-    const f32 dy = y < r.y ? r.y - y : (y >= r.bottom() ? y - r.bottom() : 0.0f);
-    const f32 distance = dx * dx + dy * dy;
+    // In double: in f32 the dy^2 term can be swallowed by a large dx^2 (pointer far outside).
+    const double dx = x < r.x ? double(r.x) - x : (x >= r.right() ? double(x) - r.right() : 0.0);
+    const double dy = y < r.y ? double(r.y) - y : (y >= r.bottom() ? double(y) - r.bottom() : 0.0);
+    const double distance = dx * dx + dy * dy;
     if (distance < best) {
       best = distance;
       row = i;
-      if (distance == 0.0f) break;  // inside this cell: nothing can be nearer
+      if (distance == 0.0) break;  // inside this cell: nothing can be nearer
     }
   }
   return row;