Perf: Use backend's complete_tag() for tag autocomplete (#4626)

The tag completer was loading all tags into a QStringListModel upfront
and relying on Qt's MatchContains to filter them on every keystroke.
With large collections, this caused lags on keypress.

Instead, use the Rust backend's complete_tag() in splitPath() to fetch a
limited set of pre-filtered matches on each keystroke, updating the
model dynamically. This is the same call already used by the Svelte tag
editor in the Note Editor, which displays completions
near-instantaneously.

Fixes #3217

⚠️ Code generated by Claude Sonnet 4.6

Manually tested and confirmed to be working as intended.

Author: user1823

qt/aqt/tagedit.py

@@ -4,13 +4,15 @@
 from __future__ import annotations
 
 import re
-from collections.abc import Iterable
+from typing import cast
 
 from anki.collection import Collection
 from aqt import gui_hooks
 from aqt.qt import *
 from aqt.qt import sip
 
+TAG_COMPLETIONS_LIMIT = 500
+
 
 class TagEdit(QLineEdit):
     _completer: QCompleter | TagCompleter
@@ -29,18 +31,16 @@ def __init__(self, parent: QWidget, type: int = 0) -> None:
             self._completer = QCompleter(self.model, parent)
         self._completer.setCompletionMode(QCompleter.CompletionMode.PopupCompletion)
         self._completer.setCaseSensitivity(Qt.CaseSensitivity.CaseInsensitive)
-        self._completer.setFilterMode(Qt.MatchFlag.MatchContains)
         self.setCompleter(self._completer)
 
     def setCol(self, col: Collection) -> None:
         "Set the current col, updating list of available tags."
         self.col = col
-        l: Iterable[str]
         if self.type == 0:
-            l = self.col.tags.all()
+            pass
         else:
-            l = (d.name for d in self.col.decks.all_names_and_ids())
-        self.model.setStringList(l)
+            l = list(d.name for d in self.col.decks.all_names_and_ids())
+            self.model.setStringList(l)
 
     def focusInEvent(self, evt: QFocusEvent | None) -> None:
         QLineEdit.focusInEvent(self, evt)
@@ -137,7 +137,12 @@ def splitPath(self, tags: str | None) -> list[str]:
             self.cursor = len(self.tags) - 1
         else:
             self.cursor = stripped_tags.count(" ", 0, p)
-        return [self.tags[self.cursor]]
+        current_tag = self.tags[self.cursor]
+        matches = self.edit.col._backend.complete_tag(
+            input=current_tag, match_limit=TAG_COMPLETIONS_LIMIT
+        )
+        cast(QStringListModel, self.model()).setStringList(list(matches))
+        return [""]
 
     def pathFromIndex(self, idx: QModelIndex) -> str:
         if self.cursor is None: