fix(ui): eliminate sidebar Filters flash on tab switch

src/components/issues/IssuesList.tsx

@@ -1,4 +1,10 @@
-import React, { useCallback, useEffect, useMemo, useState } from 'react';
+import React, {
+  useCallback,
+  useEffect,
+  useLayoutEffect,
+  useMemo,
+  useState,
+} from 'react';
 import {
   DebouncedSearchInput,
   useDebouncedSearchDraft,
@@ -272,10 +278,23 @@ const IssuesList: React.FC<IssuesListProps> = ({
   );
 
   const isLargeScreen = useMediaQuery(theme.breakpoints.up('xl'));
-  const [portalTarget, setPortalTarget] = useState<HTMLElement | null>(null);
-  useEffect(() => {
-    setPortalTarget(document.getElementById('tabs-options-portal'));
-  }, []);
+  // Seed synchronously so tab/filter re-mounts don't flash an empty Filters
+  // slot in the sidebar between the first render and the post-paint effect.
+  // useLayoutEffect handles the rare initial-mount miss before paint.
+  const [portalTarget, setPortalTarget] = useState<HTMLElement | null>(() =>
+    typeof document === 'undefined'
+      ? null
+      : document.getElementById('tabs-options-portal'),
+  );
+  // Always validate (don't just skip when set) — on cross-page navigation
+  // the useState initializer runs while the previous page's
+  // #tabs-options-portal is still attached, so portalTarget can be a stale
+  // node that gets removed at commit. Re-resolve before paint.
+  useLayoutEffect(() => {
+    const current = document.getElementById('tabs-options-portal');
+    if (current === portalTarget) return;
+    setPortalTarget(current);
+  }, [portalTarget]);
 
   const [optionsAnchorEl, setOptionsAnchorEl] = useState<HTMLElement | null>(
     null,

src/components/leaderboard/TopMinersTable.tsx

@@ -1,6 +1,7 @@
 import React, {
   useCallback,
   useEffect,
+  useLayoutEffect,
   useMemo,
   useState,
   useRef,
@@ -378,7 +379,14 @@ const TopMinersTable: React.FC<TopMinersTableProps> = ({
   }, [filteredMiners.length, visibleCount, setVisibleCount]);
 
   const isLargeScreen = useMediaQuery(theme.breakpoints.up('xl'));
-  const [portalTarget, setPortalTarget] = useState<HTMLElement | null>(null);
+  // Seed synchronously so leaderboard timeline/filter switches don't flash
+  // an empty Filters slot in the sidebar. useLayoutEffect catches the rare
+  // initial-mount miss before paint.
+  const [portalTarget, setPortalTarget] = useState<HTMLElement | null>(() =>
+    typeof document === 'undefined'
+      ? null
+      : document.getElementById('tabs-options-portal'),
+  );
   const observerTarget = useRef<HTMLDivElement>(null);
   const [stackedLayoutPage, setStackedLayoutPage] = useState(0);
 
@@ -441,9 +449,15 @@ const TopMinersTable: React.FC<TopMinersTableProps> = ({
     />
   ) : null;
 
-  useEffect(() => {
-    setPortalTarget(document.getElementById('tabs-options-portal'));
-  }, []);
+  // Always validate (don't just skip when set) — on cross-page navigation
+  // the useState initializer runs while the previous page's
+  // #tabs-options-portal is still attached, so portalTarget can be a stale
+  // node that gets removed at commit. Re-resolve before paint.
+  useLayoutEffect(() => {
+    const current = document.getElementById('tabs-options-portal');
+    if (current === portalTarget) return;
+    setPortalTarget(current);
+  }, [portalTarget]);
 
   useEffect(() => {
     if (!isLargeScreen || !observerTarget.current || remainingMiners <= 0) {

src/pages/WatchlistPage.tsx

@@ -1,6 +1,7 @@
 import React, {
   useCallback,
   useEffect,
+  useLayoutEffect,
   useMemo,
   useState,
   useRef,
@@ -572,12 +573,27 @@ const OptionsLabel: React.FC<{ children: React.ReactNode }> = ({
 
 /* ─── WatchlistPortal: sidebar panel on xl, popover button otherwise ─── */
 const WatchlistPortal: React.FC<WatchlistOptionsButtonProps> = (props) => {
-  const [target, setTarget] = useState<HTMLElement | null>(null);
+  // Seed from the DOM synchronously so on tab switches (where the parent's
+  // #tabs-options-portal Box is already in the document) the first render
+  // already returns the Portal — no intermediate "fallback" paint, no flash
+  // of an empty Filters slot in the sidebar.
+  const [target, setTarget] = useState<HTMLElement | null>(() =>
+    typeof document === 'undefined'
+      ? null
+      : document.getElementById('tabs-options-portal'),
+  );
   const isLargeScreen = useMediaQuery(theme.breakpoints.up('xl'));
 
-  useEffect(() => {
-    setTarget(document.getElementById('tabs-options-portal'));
-  }, []);
+  // Always validate (don't just skip when set) — on cross-page navigation
+  // the useState initializer runs while the OLD page's #tabs-options-portal
+  // is still attached, so `target` can be a stale node that gets removed at
+  // commit. useLayoutEffect re-resolves before paint and replaces the
+  // stale node with the current page's portal target.
+  useLayoutEffect(() => {
+    const current = document.getElementById('tabs-options-portal');
+    if (current === target) return;
+    setTarget(current);
+  }, [target]);
 
   if (target && isLargeScreen) {
     return (