Refine minimap variable highlighting logic

The updated logic now correctly handles four distinct cases: variables
used by the selected cell, variables declared by the selected cell,
variables flowing through from ancestors, and variables in downstream
cells that will be re-executed. This provides more precise visual
feedback about which variables are actually affected by cell selection
in the minimap.

Author: manzt

frontend/src/components/editor/chrome/wrapper/floating-minimap.tsx

@@ -8,7 +8,10 @@ import type { CellId } from "@/core/cells/ids";
 import { useVariables } from "@/core/variables/state";
 import { cn } from "@/utils/cn";
 import { invariant } from "@/utils/invariant";
-import { cellGraphsAtom, isVariableInSelectedDataflow } from "./minimap-state";
+import {
+  cellGraphsAtom,
+  isVariableAffectedBySelectedCell,
+} from "./minimap-state";
 
 interface MinimapCellProps {
   cellId: CellId;
@@ -191,7 +194,7 @@ const MinimapCell: React.FC<MinimapCellProps> = (props) => {
                         !isSelected &&
                         selectedCellId &&
                         selectedGraph &&
-                        isVariableInSelectedDataflow(variable, {
+                        isVariableAffectedBySelectedCell(variable, {
                           cellId: selectedCellId,
                           graph: selectedGraph,
                         }),

frontend/src/components/editor/chrome/wrapper/minimap-state.ts

@@ -119,41 +119,51 @@ export const cellGraphsAtom = atom((get) => {
   return buildCellGraph(notebook.cellIds.inOrderIds, variables);
 });
 
-export function isVariableInSelectedDataflow(
+/**
+ * Determines if a variable should be highlighted when a cell is selected.
+ *
+ * For upstream dataflow: We can precisely track which variables flow INTO the selected cell
+ * by checking what the selected cell actually uses.
+ *
+ * For downstream dataflow: We only have cell-level dependency information, not variable-level,
+ * so we highlight ALL variables in descendant cells since they will all be re-executed
+ * when the selected cell changes.
+ *
+ * @param variable - The variable to check for highlighting
+ * @param selected - The currently selected cell and its dependency graph
+ */
+export function isVariableAffectedBySelectedCell(
   variable: Variable,
-  selected: {
-    cellId: CellId;
-    graph: {
-      ancestors: ReadonlySet<CellId>;
-      descendants: ReadonlySet<CellId>;
-    };
-  },
+  selected: { cellId: CellId; graph: CellGraphNode },
 ): boolean {
-  // Variable is used by the selected cell
+  // Case 1: Variable is used by the selected cell (incoming dataflow)
   if (variable.usedBy.includes(selected.cellId)) {
     return true;
   }
 
-  // Variable is declared by the selected cell and used by someone
-  if (
-    variable.declaredBy.includes(selected.cellId) &&
-    variable.usedBy.length > 0
-  ) {
+  // Case 2: Variable is declared by the selected cell (outgoing dataflow)
+  if (variable.declaredBy.includes(selected.cellId)) {
     return true;
   }
 
-  // Variable flows through the selected cell:
-  // It must be declared by an ancestor AND used by the selected cell or a descendant
+  // Case 3: Variable is declared by an ancestor AND used by selected cell or descendants
+  // This captures the flow-through case
   const isDeclaredByAncestor = variable.declaredBy.some((declarer) =>
-    selected.graph.ancestors.has(declarer),
+    selected.graph.parents.has(declarer),
   );
 
   if (isDeclaredByAncestor) {
-    // Only highlight if this variable is actually used by selected cell or its descendants
-    return (
-      variable.usedBy.includes(selected.cellId) ||
-      variable.usedBy.some((user) => selected.graph.descendants.has(user))
-    );
+    return variable.usedBy.some((user) => selected.graph.parents.has(user));
+  }
+
+  // Case 4: Variable is declared by a descendant
+  // These will be re-executed when the selected cell changes
+  const isDeclaredByDescendant = variable.declaredBy.some((declarer) =>
+    selected.graph.children.has(declarer),
+  );
+
+  if (isDeclaredByDescendant) {
+    return true;
   }
 
   return false;