Improve initial directory listing latency
Avoid synchronous child counts during item construction, use extension-based MIME detection for initial scans, and defer expensive MIME icon resolution by using generic file icons for ordinary files. Document the local xdg-desktop-portal FileChooser workaround for COSMIC portal crashes.
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docs/local-performance-and-portal-notes.md
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docs/local-performance-and-portal-notes.md
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# Local performance and portal notes
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Date: 2026-05-05
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This repository carries a local patch aimed at improving initial directory
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display latency in large folders, plus a system-level workaround for a COSMIC
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portal FileChooser crash observed with Firefox and Chromium.
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## Directory listing latency
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The slow path was the initial construction of `Item` values in `src/tab.rs`.
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On a test folder with about 2000 entries, raw filesystem enumeration and stat
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calls completed in a few milliseconds, while COSMIC Files took multiple seconds
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before showing the directory.
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The local patch keeps initial item construction cheap:
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- directory child counts are no longer computed synchronously in
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`item_from_entry` and `item_from_gvfs_info`;
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- initial MIME detection uses extension-based `mime_guess`;
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- regular files use a generic file icon during the initial scan instead of
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resolving the full MIME icon immediately.
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This keeps folders and `.desktop` files special-cased, while avoiding expensive
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per-file work for ordinary files during first paint.
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Measured locally during investigation:
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- before the MIME/icon changes: about 3.1 seconds for `~/Téléchargements`;
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- after avoiding full MIME icon resolution during scan: below the temporary
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100 ms perf-log threshold for the same folder.
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## File chooser portal workaround
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Firefox and Chromium were failing to open `Save As` on the first attempt because
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`xdg-desktop-portal-cosmic` crashed while handling `FileChooser`.
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Logs showed:
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```text
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Backend call failed: Remote peer disconnected
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xdg-desktop-portal-cosmic ... status=11/SEGV
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```
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The working local system workaround is to remove
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`org.freedesktop.impl.portal.FileChooser` from:
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```text
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/usr/share/xdg-desktop-portal/portals/cosmic.portal
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```
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so the file chooser falls back to GTK. The resulting local `cosmic.portal`
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keeps COSMIC for the other interfaces:
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```ini
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[portal]
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DBusName=org.freedesktop.impl.portal.desktop.cosmic
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Interfaces=org.freedesktop.impl.portal.Access;org.freedesktop.impl.portal.Screenshot;org.freedesktop.impl.portal.Settings;org.freedesktop.impl.portal.ScreenCast
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UseIn=COSMIC
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```
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Backups created locally:
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```text
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/usr/share/xdg-desktop-portal/portals/cosmic.portal.bak-codex-20260505-filechooser
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/usr/share/xdg-desktop-portal/cosmic-portals.conf.bak-codex-20260505
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/usr/share/xdg-desktop-portal/cosmic-portals.conf.bak-codex-20260505-2
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```
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After editing portal files, restart:
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```sh
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systemctl --user restart xdg-desktop-portal.service xdg-desktop-portal-gtk.service
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```
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Package updates may overwrite `/usr/share/xdg-desktop-portal/portals/cosmic.portal`.
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If `Save As` starts needing two attempts again, re-check that `FileChooser` has
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not been reintroduced in `cosmic.portal`.
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72
src/tab.rs
72
src/tab.rs
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@ -302,6 +302,26 @@ pub fn folder_icon_symbolic(path: &PathBuf, icon_size: u16) -> widget::icon::Han
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.handle()
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}
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fn generic_file_icons(
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sizes: IconSizes,
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) -> (
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widget::icon::Handle,
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widget::icon::Handle,
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widget::icon::Handle,
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) {
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(
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widget::icon::from_name("text-x-generic")
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.size(sizes.grid())
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.handle(),
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widget::icon::from_name("text-x-generic")
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.size(sizes.list())
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.handle(),
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widget::icon::from_name("text-x-generic")
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.size(sizes.list_condensed())
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.handle(),
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)
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}
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//TODO: replace with Path::has_trailing_sep when stable
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fn has_trailing_sep(path: &Path) -> bool {
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path.as_os_str()
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@ -665,9 +685,9 @@ pub fn item_from_gvfs_info(path: PathBuf, file_info: gio::FileInfo, sizes: IconS
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folder_icon(&path, sizes.list_condensed()),
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)
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} else {
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// ALWAYS assume we're remote for mime guessing here, since gvfs reading can be expensive
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// @todo - expose this as a config option?
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let mime = mime_for_path(&path, None, true);
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// Keep the initial directory scan cheap. Opening files still
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// recalculates MIME from the real path before launching apps.
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let mime = mime_guess::from_path(&path).first_or_octet_stream();
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//TODO: clean this up, implement for trash
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let icon_name_opt = if mime == "application/x-desktop" {
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@ -684,28 +704,21 @@ pub fn item_from_gvfs_info(path: PathBuf, file_info: gio::FileInfo, sizes: IconS
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desktop_icon_handle(&icon_name, sizes.list_condensed()),
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)
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} else {
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let (icon_handle_grid, icon_handle_list, icon_handle_list_condensed) =
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generic_file_icons(sizes);
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(
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mime.clone(),
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mime_icon(mime.clone(), sizes.grid()),
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mime_icon(mime.clone(), sizes.list()),
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mime_icon(mime, sizes.list_condensed()),
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mime,
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icon_handle_grid,
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icon_handle_list,
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icon_handle_list_condensed,
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)
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}
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};
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let mut children_opt = None;
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let children_opt = None;
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let mut dir_size = DirSize::NotDirectory;
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if is_dir && !remote {
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dir_size = DirSize::Calculating(Controller::default());
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//TODO: calculate children in the background (and make it cancellable?)
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match fs::read_dir(&path) {
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Ok(entries) => {
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children_opt = Some(entries.count());
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}
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Err(err) => {
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log::warn!("failed to read directory {}: {}", path.display(), err);
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}
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}
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}
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let display_name = display_name_for_file(&path, &file_info.display_name(), false, is_desktop);
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@ -807,7 +820,9 @@ pub fn item_from_entry(
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folder_icon(&path, sizes.list_condensed()),
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)
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} else {
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let mime = mime_for_path(&path, Some(&metadata), remote);
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// Keep the initial directory scan cheap. Opening files still
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// recalculates MIME from the real path before launching apps.
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let mime = mime_guess::from_path(&path).first_or_octet_stream();
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//TODO: clean this up, implement for trash
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let icon_name_opt = if mime == "application/x-desktop" {
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is_desktop = true;
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@ -823,28 +838,21 @@ pub fn item_from_entry(
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desktop_icon_handle(&icon_name, sizes.list_condensed()),
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)
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} else {
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let (icon_handle_grid, icon_handle_list, icon_handle_list_condensed) =
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generic_file_icons(sizes);
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(
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mime.clone(),
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mime_icon(mime.clone(), sizes.grid()),
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mime_icon(mime.clone(), sizes.list()),
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mime_icon(mime, sizes.list_condensed()),
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mime,
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icon_handle_grid,
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icon_handle_list,
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icon_handle_list_condensed,
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)
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}
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};
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let mut children_opt = None;
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let children_opt = None;
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let mut dir_size = DirSize::NotDirectory;
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if metadata.is_dir() && !remote {
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dir_size = DirSize::Calculating(Controller::default());
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//TODO: calculate children in the background (and make it cancellable?)
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match fs::read_dir(&path) {
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Ok(entries) => {
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children_opt = Some(entries.count());
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}
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Err(err) => {
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log::warn!("failed to read directory {}: {}", path.display(), err);
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}
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}
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}
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let display_name = display_name_for_file(&path, &name, is_gvfs, is_desktop);
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