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.

docs/local-performance-and-portal-notes.md

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+# Local performance and portal notes
+
+Date: 2026-05-05
+
+This repository carries a local patch aimed at improving initial directory
+display latency in large folders, plus a system-level workaround for a COSMIC
+portal FileChooser crash observed with Firefox and Chromium.
+
+## Directory listing latency
+
+The slow path was the initial construction of `Item` values in `src/tab.rs`.
+On a test folder with about 2000 entries, raw filesystem enumeration and stat
+calls completed in a few milliseconds, while COSMIC Files took multiple seconds
+before showing the directory.
+
+The local patch keeps initial item construction cheap:
+
+- directory child counts are no longer computed synchronously in
+  `item_from_entry` and `item_from_gvfs_info`;
+- initial MIME detection uses extension-based `mime_guess`;
+- regular files use a generic file icon during the initial scan instead of
+  resolving the full MIME icon immediately.
+
+This keeps folders and `.desktop` files special-cased, while avoiding expensive
+per-file work for ordinary files during first paint.
+
+Measured locally during investigation:
+
+- before the MIME/icon changes: about 3.1 seconds for `~/Téléchargements`;
+- after avoiding full MIME icon resolution during scan: below the temporary
+  100 ms perf-log threshold for the same folder.
+
+## File chooser portal workaround
+
+Firefox and Chromium were failing to open `Save As` on the first attempt because
+`xdg-desktop-portal-cosmic` crashed while handling `FileChooser`.
+
+Logs showed:
+
+```text
+Backend call failed: Remote peer disconnected
+xdg-desktop-portal-cosmic ... status=11/SEGV
+```
+
+The working local system workaround is to remove
+`org.freedesktop.impl.portal.FileChooser` from:
+
+```text
+/usr/share/xdg-desktop-portal/portals/cosmic.portal
+```
+
+so the file chooser falls back to GTK. The resulting local `cosmic.portal`
+keeps COSMIC for the other interfaces:
+
+```ini
+[portal]
+DBusName=org.freedesktop.impl.portal.desktop.cosmic
+Interfaces=org.freedesktop.impl.portal.Access;org.freedesktop.impl.portal.Screenshot;org.freedesktop.impl.portal.Settings;org.freedesktop.impl.portal.ScreenCast
+UseIn=COSMIC
+```
+
+Backups created locally:
+
+```text
+/usr/share/xdg-desktop-portal/portals/cosmic.portal.bak-codex-20260505-filechooser
+/usr/share/xdg-desktop-portal/cosmic-portals.conf.bak-codex-20260505
+/usr/share/xdg-desktop-portal/cosmic-portals.conf.bak-codex-20260505-2
+```
+
+After editing portal files, restart:
+
+```sh
+systemctl --user restart xdg-desktop-portal.service xdg-desktop-portal-gtk.service
+```
+
+Package updates may overwrite `/usr/share/xdg-desktop-portal/portals/cosmic.portal`.
+If `Save As` starts needing two attempts again, re-check that `FileChooser` has
+not been reintroduced in `cosmic.portal`.