diff --git a/src/ui.cpp b/src/ui.cpp
index 32d8625..80e010c 100644
--- a/src/ui.cpp
+++ b/src/ui.cpp
@@ -3419,6 +3419,20 @@ double math_set_scale(MathDialog* d, double want) {
   return d->view_scale;
 }
 
+/// Zooms about a canvas position: one step in for `dy < 0`, one out otherwise. The graph point
+/// under `(px, py)` is still under it afterwards -- which is what makes the canvas follow the
+/// hand rather than slide away from it.
+void math_zoom_at(MathDialog* d, double px, double py, double dy) {
+  double gx = 0.0;
+  double gy = 0.0;
+  math_to_graph(d, px, py, gx, gy);
+  const double s = math_set_scale(d, d->view_scale * (dy < 0.0 ? kMathZoomStep
+                                                              : 1.0 / kMathZoomStep));
+  d->view_x = px - gx * s;
+  d->view_y = py - gy * s;
+  gtk_widget_queue_draw(d->canvas);
+}
+
 void math_view_reset(MathDialog* d) {
   math_set_scale(d, 1.0);
   d->view_x = 0.0;
@@ -3625,9 +3639,9 @@ void math_canvas_drag(GtkGestureDrag*, double dx, double dy, gpointer user) {
 gboolean math_canvas_scrolled(GtkEventControllerScroll* scroll, double, double dy, gpointer user) {
   MathDialog* d = static_cast<MathDialog*>(user);
   if (dy == 0.0) return TRUE;
-  // About the pointer: the graph point under it must still be under it afterwards, or the canvas
-  // slides away from the hand. With no controller (the headless probe calls this directly) there
-  // is no pointer to read, so the canvas centre is the anchor.
+  // Resolve the anchor: the pointer position from the current event, or the canvas centre as a
+  // fallback (a real scroll event always arrives on an allocated widget, so the fallback is only
+  // reached in degenerate cases).
   double px = 0.0;
   double py = 0.0;
   bool have = false;
@@ -3636,21 +3650,12 @@ gboolean math_canvas_scrolled(GtkEventControllerScroll* scroll, double, double d
     if (event != nullptr) have = gdk_event_get_position(event, &px, &py);
   }
   if (!have) {
-    double w = d->canvas != nullptr ? gtk_widget_get_width(d->canvas) : 0;
-    double h = d->canvas != nullptr ? gtk_widget_get_height(d->canvas) : 0;
-    if (w <= 0.0) w = 420.0;
-    if (h <= 0.0) h = 300.0;
-    px = w * 0.5;
-    py = h * 0.5;
+    const double w = d->canvas != nullptr ? static_cast<double>(gtk_widget_get_width(d->canvas)) : 0.0;
+    const double h = d->canvas != nullptr ? static_cast<double>(gtk_widget_get_height(d->canvas)) : 0.0;
+    px = w > 0.0 ? w * 0.5 : 0.0;
+    py = h > 0.0 ? h * 0.5 : 0.0;
   }
-  double gx = 0.0;
-  double gy = 0.0;
-  math_to_graph(d, px, py, gx, gy);
-  const double s = math_set_scale(d, d->view_scale * (dy < 0.0 ? kMathZoomStep
-                                                              : 1.0 / kMathZoomStep));
-  d->view_x = px - gx * s;
-  d->view_y = py - gy * s;
-  gtk_widget_queue_draw(d->canvas);
+  math_zoom_at(d, px, py, dy);
   return TRUE;
 }
 
@@ -6974,8 +6979,8 @@ gboolean math_probe(gpointer user) {
       double gx0 = 0.0;
       double gy0 = 0.0;
       math_to_graph(v, px, py, gx0, gy0);
-      math_canvas_scrolled(nullptr, 0.0, -1.0, v);
-      math_canvas_scrolled(nullptr, 0.0, -1.0, v);
+      math_zoom_at(v, px, py, -1.0);
+      math_zoom_at(v, px, py, -1.0);
       double gx1 = 0.0;
       double gy1 = 0.0;
       math_to_graph(v, px, py, gx1, gy1);