fix(desktop): stabilize members dialog scrolling (#6670)

## Summary

- Keep virtualized member rows measurable by removing
`content-visibility: auto` from the measured row subtree.
- Use the member card's 60px baseline as the virtualizer estimate while
retaining deferred rendering for eager search and archived-member lists.
- Cover large rosters with a regression test that checks stable scroll
extent across the list and verifies the final member remains reachable.

### Related issue

None found.

### Testing

- `just ci`
- `pnpm -C desktop build:e2e`
- `pnpm -C desktop exec playwright test tests/e2e/channels.spec.ts
--grep 'members sidebar virtualizes large channel rosters'
--repeat-each=5`

#### Before


https://github.com/user-attachments/assets/a5fcc040-6872-4200-bcc3-7b4197a4dd23

#### After


https://github.com/user-attachments/assets/f2c97f2f-3719-4c2a-b17a-2450c6c70a55

Signed-off-by: Matt Toohey <contact@matttoohey.com>

desktop/src/features/channels/ui/MembersSidebar.tsx

@@ -70,6 +70,7 @@ import { useMembersSidebarActions } from "./useMembersSidebarActions";
 import { useMembersSidebarModeration } from "./useMembersSidebarModeration";
 const MEMBER_ADD_RESULT_LIMIT = 50;
 const MEMBER_SEARCH_MIN_QUERY_LENGTH = 2;
+const MEMBER_ROW_ESTIMATE_PX = 60;
 type AddMemberSearchCandidate = UserSearchResult & {
   isManagedAgent?: boolean;
   isMember?: boolean;
@@ -645,57 +646,62 @@ export function MembersSidebar({
         ? managedAgentPairAction(managedAgentRuntime)
         : undefined;
     return (
-      <div className="content-visibility-auto" key={member.pubkey}>
-        <MembersSidebarMemberCard
-          canChangeRole={canManageMembers && member.pubkey !== currentPubkey}
-          canModerate={canModerate && member.pubkey !== currentPubkey}
-          canRemoveMember={canRemoveMember(member)}
-          isActionPending={
-            isActionPending ||
-            changeRoleMutation.isPending ||
-            isModerationPending
-          }
-          isArchived={isArchived}
-          managedAgent={managedAgent}
-          managedAgentRuntime={managedAgentRuntime}
-          member={member}
-          memberIsBot={memberIsBot}
-          memberAvatarLabel={
-            member.displayName ?? truncatePubkey(member.pubkey)
-          }
-          memberLabel={formatMemberName(member, currentPubkey)}
-          moderationState={moderationStateByPubkey.get(
-            normalizePubkey(member.pubkey),
-          )}
-          onBan={onBan}
-          onChangeRole={(m, role) => {
-            void changeRoleMutation.mutateAsync({ pubkey: m.pubkey, role });
-          }}
-          onEditRespondTo={memberIsBot ? setEditRespondToAgent : undefined}
-          onManagedAgentAction={(agent) => {
-            void handleAgentLifecycleAction(agent, managedAgentRuntime);
-          }}
-          onOpenProfile={handleOpenProfile}
-          onRemoveMember={handleRemoveMember}
-          onTimeout={onTimeout}
-          onUnban={onUnban}
-          onUntimeout={onUntimeout}
-          onViewActivity={
-            onViewActivity
-              ? (pubkey: string) => {
-                  onOpenChange(false);
-                  onViewActivity(pubkey);
-                }
-              : undefined
-          }
-          pairAction={pairAction}
-          presenceStatus={
-            memberPresenceQuery.data?.[member.pubkey.toLowerCase()] ?? null
-          }
-          profileAvatarUrl={memberProfile?.avatarUrl ?? null}
-          showOtherSetupMarker={showOtherSetupMarker}
-          viewerIsOwner={viewerIsOwner}
-        />
+      <MembersSidebarMemberCard
+        canChangeRole={canManageMembers && member.pubkey !== currentPubkey}
+        canModerate={canModerate && member.pubkey !== currentPubkey}
+        canRemoveMember={canRemoveMember(member)}
+        isActionPending={
+          isActionPending || changeRoleMutation.isPending || isModerationPending
+        }
+        isArchived={isArchived}
+        managedAgent={managedAgent}
+        managedAgentRuntime={managedAgentRuntime}
+        member={member}
+        memberIsBot={memberIsBot}
+        memberAvatarLabel={member.displayName ?? truncatePubkey(member.pubkey)}
+        memberLabel={formatMemberName(member, currentPubkey)}
+        moderationState={moderationStateByPubkey.get(
+          normalizePubkey(member.pubkey),
+        )}
+        onBan={onBan}
+        onChangeRole={(m, role) => {
+          void changeRoleMutation.mutateAsync({ pubkey: m.pubkey, role });
+        }}
+        onEditRespondTo={memberIsBot ? setEditRespondToAgent : undefined}
+        onManagedAgentAction={(agent) => {
+          void handleAgentLifecycleAction(agent, managedAgentRuntime);
+        }}
+        onOpenProfile={handleOpenProfile}
+        onRemoveMember={handleRemoveMember}
+        onTimeout={onTimeout}
+        onUnban={onUnban}
+        onUntimeout={onUntimeout}
+        onViewActivity={
+          onViewActivity
+            ? (pubkey: string) => {
+                onOpenChange(false);
+                onViewActivity(pubkey);
+              }
+            : undefined
+        }
+        pairAction={pairAction}
+        presenceStatus={
+          memberPresenceQuery.data?.[member.pubkey.toLowerCase()] ?? null
+        }
+        profileAvatarUrl={memberProfile?.avatarUrl ?? null}
+        showOtherSetupMarker={showOtherSetupMarker}
+        viewerIsOwner={viewerIsOwner}
+      />
+    );
+  }
+
+  function renderDeferredMemberCard(
+    member: ChannelMember,
+    memberIsBot: boolean,
+  ) {
+    return (
+      <div className="content-visibility-auto-member-row" key={member.pubkey}>
+        {renderMemberCard(member, memberIsBot)}
       </div>
     );
   }
@@ -780,7 +786,7 @@ export function MembersSidebar({
                 {normalizedSearchQuery ? (
                   <div>
                     {filteredActiveMembers.map((member) =>
-                      renderMemberCard(member, isBot(member)),
+                      renderDeferredMemberCard(member, isBot(member)),
                     )}
                     {canAddMembers ? (
                       <>
@@ -824,6 +830,7 @@ export function MembersSidebar({
                 ) : filteredActiveMembers.length > 0 ? (
                   <VirtualizedList
                     className="h-[calc(100%_-_2.25rem)]"
+                    estimateSize={MEMBER_ROW_ESTIMATE_PX}
                     getItemKey={(member) => member.pubkey}
                     items={filteredActiveMembers}
                     renderItem={(member) =>
@@ -867,7 +874,7 @@ export function MembersSidebar({
                     data-testid="members-sidebar-archived-list"
                   >
                     {filteredArchivedMembers.map((member) =>
-                      renderMemberCard(member, isBot(member)),
+                      renderDeferredMemberCard(member, isBot(member)),
                     )}
                     {filteredArchivedMembers.length === 0 ? (
                       <p className="text-sm text-muted-foreground">

desktop/src/shared/styles/globals/utilities.css

@@ -37,6 +37,15 @@
     contain-intrinsic-size: auto 2rem;
   }
 
+  /*
+   * Member cards rendered eagerly can skip offscreen layout and paint.
+   * Virtualized member rows must not use this utility.
+   */
+  .content-visibility-auto-member-row {
+    content-visibility: auto;
+    contain-intrinsic-size: auto 3.75rem;
+  }
+
   .buzz-huddle-tooltip {
     background: hsl(
       var(--huddle-tooltip-surface, var(--huddle-control-surface, 0 0% 20%))

desktop/tests/e2e/channels.spec.ts

@@ -4007,7 +4007,40 @@ test("members sidebar virtualizes large channel rosters", async ({ page }) => {
   await expect(memberRows.first()).toBeVisible();
   expect(await memberRows.count()).toBeLessThan(50);
 
+  const firstGeneratedRow = memberList.getByTestId(
+    `sidebar-member-${pubkeys[0]}`,
+  );
+  await expect
+    .poll(() =>
+      firstGeneratedRow.evaluate(
+        (row) =>
+          getComputedStyle(row.parentElement as HTMLElement).contentVisibility,
+      ),
+    )
+    .toBe("visible");
+
   const virtualizedList = memberList.locator(".overflow-y-auto");
+  await page.evaluate(() => document.fonts.ready);
+
+  const heights: number[] = [];
+  for (const ratio of [0, 0.1, 0.25, 0.5, 0.75, 1]) {
+    heights.push(
+      await virtualizedList.evaluate(async (element, ratio) => {
+        element.scrollTop =
+          (element.scrollHeight - element.clientHeight) * ratio;
+        element.dispatchEvent(new Event("scroll"));
+
+        await new Promise<void>((resolve) =>
+          requestAnimationFrame(() => requestAnimationFrame(() => resolve())),
+        );
+
+        return element.scrollHeight;
+      }, ratio),
+    );
+  }
+
+  expect(Math.max(...heights) - Math.min(...heights)).toBeLessThan(120);
+
   await virtualizedList.evaluate((element) => {
     element.scrollTop = element.scrollHeight;
     element.dispatchEvent(new Event("scroll"));