Fix: silence get_file_at FE/BE drift warning during async listing refresh

The 'frontend/backend index mismatch!' log_error in get_file_at was added as
diagnostic to catch genuine drift, but it fires legitimately during the brief
window between a directory-diff event arriving and the FE's cached
`totalCount` being updated by the subsequent `getTotalCount` IPC. The user
saw it in their move logs (cursor settled on index 29 in a listing that had
already shrunk to 29 entries) — visually nothing was broken (FE handles
`null` from getFileAt gracefully), but the log was noisy and triggered the
opt-in error reporter.

Two-pronged fix:

- **Backend** (`get_file_at`): demote the out-of-bounds path from
  `crate::log_error!` (auto-reportable) to `log::debug!`. The breadcrumb is
  still in the file log when investigating cursor/selection bugs, but it no
  longer surfaces as an error to the user or in error reports. Updated the
  comment to explain the legitimate drift case.

- **Frontend** (`buildMcpFileList` in FilePane.svelte): the loop iterating
  over the visible range to sync MCP context state used to skip null
  entries silently and continue. That meant for a backendStart=0,
  visibleRangeEnd=30 against a listing that had shrunk to 29, we'd make 30
  IPC calls — only the last one out of bounds, but every iteration still
  paid the round-trip. Changed `if (entry) {push}` to `if (!entry) break`:
  null means the BE listing has fewer entries than our cached `totalCount`
  (a directory-diff is mid-flight), so stop iterating instead of paying for
  futile lookups.

Tests: 1606 Rust, 1739 Svelte, clippy/svelte-check/eslint all green.

Author: vdavid

apps/desktop/src-tauri/src/file_system/listing/operations.rs

@@ -243,10 +243,14 @@ pub fn get_file_at(listing_id: &str, index: usize, include_hidden: bool) -> Resu
     let result = visible_entries(&listing.entries, include_hidden).nth(index).cloned();
     if result.is_none() {
         let total = visible_entries(&listing.entries, include_hidden).count();
-        // FE/BE index desync surfaces as broken cursor/selection. Use `log_error!` so
-        // opt-in users help us catch the trigger conditions.
-        crate::log_error!(
-            "get_file_at: index {} out of bounds (listing {} has {} entries at {}) - frontend/backend index mismatch!",
+        // Out-of-bounds is expected briefly after a mutation: the FE iterates over a
+        // cached `totalCount` that may lag the BE listing during the async refetch
+        // window opened by a `directory-diff` event. The FE handles `None` gracefully
+        // (skips the entry, breaks the loop). Logged at debug so we still have the
+        // breadcrumb when investigating cursor/selection bugs without firing crash
+        // reports for legitimate drift.
+        log::debug!(
+            "get_file_at: index {} out of bounds (listing {} has {} entries at {}) — likely FE/BE drift after async listing refresh",
             index,
             listing_id,
             total,

apps/desktop/src/lib/file-explorer/pane/FilePane.svelte

@@ -764,16 +764,19 @@
             const backendIndex = backendStart + i
             if (backendIndex >= totalCount) break
             const entry = await getFileAt(listingId, backendIndex, includeHidden)
-            if (entry) {
-                files.push({
-                    name: entry.name,
-                    path: entry.path,
-                    isDirectory: entry.isDirectory,
-                    size: entry.size,
-                    recursiveSize: entry.recursiveSize,
-                    modified: entry.modifiedAt ? new Date(entry.modifiedAt * 1000).toISOString() : undefined,
-                })
-            }
+            // Null means the listing on the BE has fewer entries than our cached
+            // `totalCount` (a directory-diff is mid-flight). Stop here — keeps the
+            // partial MCP state consistent and avoids trailing out-of-bounds calls
+            // for the rest of the visible range.
+            if (!entry) break
+            files.push({
+                name: entry.name,
+                path: entry.path,
+                isDirectory: entry.isDirectory,
+                size: entry.size,
+                recursiveSize: entry.recursiveSize,
+                modified: entry.modifiedAt ? new Date(entry.modifiedAt * 1000).toISOString() : undefined,
+            })
         }
         return files
     }