fix(input): cursors follow the pointer while dragging and snap on drop

src/plot/plot.cpp

@@ -571,7 +571,7 @@ void Plot::autoscale_axes() {
   }
 }
 
-void Plot::set_cursor_x(u32 id, f64 x) {
+void Plot::set_cursor_x(u32 id, f64 x, CursorPlacement placement) {
   if (id >= cursors_.size()) return;
   Cursor& cursor = cursors_[id];
   // `x` is the vertical cursor's coordinate and nobody else's: a ruler's `x` is dead, and
@@ -579,10 +579,26 @@ void Plot::set_cursor_x(u32 id, f64 x) {
   // cursor is (see CursorOrientation).
   if (cursor.is_horizontal()) return;
   cursor.x = x;
-  cursor.x = snapped_x(cursor);
+  // `Free` stores the value and stops: a drag moves the marker under the pointer and the
+  // snap is a separate act (`snap_cursor()`), which is what keeps the two orientations
+  // agreeing about *when* they land on their grid -- see CursorPlacement.
+  if (placement == CursorPlacement::Snapped) cursor.x = snapped_x(cursor);
   mark_measures_dirty();
 }
 
+void Plot::snap_cursor(u32 id) {
+  if (id >= cursors_.size()) return;
+  const Cursor& cursor = cursors_[id];
+  // One act, the orientation's own rule: the ruler's half of the same shape a region's
+  // `snap_region()` has. Both setters default to `Snapped`, so re-storing the coordinate
+  // the cursor already holds is what applies the rule.
+  if (cursor.is_horizontal()) {
+    set_cursor_y(id, cursor.y);
+  } else {
+    set_cursor_x(id, cursor.x);
+  }
+}
+
 void Plot::set_cursor_snap(u32 id, SnapMode mode) {
   if (id >= cursors_.size()) return;
   Cursor& cursor = cursors_[id];
@@ -595,12 +611,14 @@ void Plot::set_cursor_snap(u32 id, SnapMode mode) {
   mark_measures_dirty();
 }
 
-void Plot::set_cursor_y(u32 id, f64 y) {
+void Plot::set_cursor_y(u32 id, f64 y, CursorPlacement placement) {
   if (id >= cursors_.size()) return;
   Cursor& ruler = cursors_[id];
   if (!ruler.is_horizontal()) return;
   ruler.y = y;
-  ruler.y = snapped_y(ruler);
+  // As `set_cursor_x()`: `Free` is the drag's half of the gesture, and `snap_cursor()`
+  // commits the ruler's own rule on release.
+  if (placement == CursorPlacement::Snapped) ruler.y = snapped_y(ruler);
   // No mark_measures_dirty(), for `add_h_ruler()`'s reason: a measurement is read from two
   // x coordinates, and a ruler's y is not one of them.
 }