fix(whatif): preserve input focus while solving

Author: ApocDev

app/e2e/mut/rebalance.e2e.ts

@@ -1,6 +1,25 @@
 import { expect, test } from "@playwright/test";
 import { goto, toast, undoButton } from "./helpers";
 
+test("what-if target stays editable while the factory re-solves", async ({ page }) => {
+  await goto(page, "/whatif");
+
+  const firstDemand = page.getByRole("spinbutton").first();
+  await expect(firstDemand).toBeVisible();
+  await firstDemand.selectText();
+  await firstDemand.pressSequentially("12.34", { delay: 100 });
+
+  // Each character starts another solve. The demand list must remain mounted,
+  // preserving both the in-progress value and keyboard focus throughout them.
+  await expect(firstDemand).toBeFocused();
+  await expect(firstDemand).toHaveValue("12.34");
+  await expect(page.getByText("Block changes", { exact: false }).first()).toBeVisible({
+    timeout: 15_000,
+  });
+  await expect(firstDemand).toBeFocused();
+  await expect(firstDemand).toHaveValue("12.34");
+});
+
 /**
  * What-if "Apply all" (whole-factory re-balance): overriding a final product's
  * target surfaces the per-block changes, and Apply all commits every one of them

app/src/routes/whatif.tsx

@@ -1,5 +1,5 @@
 import { createFileRoute, Link } from "@tanstack/react-router";
-import { useQuery } from "@tanstack/react-query";
+import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { useState } from "react";
 import { factoryWhatIfFn } from "../server/factorio";
 import { Icon, IconProvider } from "../lib/icons";
@@ -43,6 +43,9 @@ function WhatIf() {
   const wf = useQuery({
     queryKey: ["whatif", overrides],
     queryFn: () => factoryWhatIfFn({ data: { demands: overrides } }),
+    // Changing a target starts a fresh whole-factory solve. Keep the demand rows
+    // mounted while it runs so the active input does not lose focus mid-edit.
+    placeholderData: keepPreviousData,
   });
   const r = wf.data;
   const changed = (r?.blocks ?? []).filter((b) => Math.abs(b.scale - 1) > SCALE_EPS);