fix(tags): complete every tag in the list, not only the first

Reported by the user: in the Edit tags fields, the first tag suggested
existing tags and the second did not. Typing a comma, a space and a
letter offered nothing.

QLineEdit::setCompleter hands completion to the line edit, which
overwrites the completer's prefix with the widget's ENTIRE text on every
keystroke. These fields hold a comma-separated list, so once one reads
"unread, fl" that whole string is matched against the tag names, nothing
matches, and completion silently stops after the first tag. Confirmed
with a probe: the prefix really is "unread, fl" and the completion count
really is zero.

Attach with setWidget instead, which keeps the popup anchored without
ceding control of the prefix, and drive it from the token under the
cursor on every edit. Setting the prefix from a textEdited handler while
leaving setCompleter in place does NOT work, which was the first attempt:
the line edit sets it again afterwards.

Accepting a candidate needed the same treatment, and is the other half of
the fix. QCompleter's own insertion replaces the whole field, so taking
"flagged" from the popup would have discarded every tag already typed.
replaceCurrentToken() overwrites only the token under the cursor and
keeps the separator's spacing, so the result is "unread, flagged" rather
than "unread,flagged".

This is the same defect QueryCompleter hit in c98b179. Having now cost
two debugging rounds, it is written into CLAUDE.md as a Qt trap rather
than a property of either class, together with the reason a test using
setText() passes against it: setText does not drive a completer at all,
so the keys have to be typed.

Both new tests were confirmed to fail against setCompleter before the fix
was kept.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/CLAUDE.md b/CLAUDE.md
index ed8faca..694852f 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -94,6 +94,21 @@ via `MainWindow::uiStatePath()`. Never write window blobs into the hand-edited c
 Build the path from `QStandardPaths::GenericStateLocation`, not `StateLocation`: the
 latter appends both the organization and the application name, and both are `qtmaildir`.
 
+**`QLineEdit::setCompleter` is wrong for any field holding more than one
+value.** It hands completion to the line edit, which then overwrites the
+completer's `completionPrefix` with the widget's **entire text** on every
+keystroke. In a field holding a list, the first value completes and nothing
+after it ever does, because "unread, fl" is matched whole against the
+candidates. Setting the prefix from a `textEdited` handler does not help: the
+line edit sets it again afterwards. Use `setCompleter` only for a field whose
+whole contents are the thing being completed; otherwise attach with
+`QCompleter::setWidget` and drive `setCompletionPrefix` and `complete()`
+yourself, and replace the token under the cursor on `activated` rather than
+letting QCompleter overwrite the field. This has been hit twice, in
+`QueryCompleter` (01ba356) and in `TagDialog`; the trap belongs to Qt, not to
+either class. A test that uses `setText()` passes against the bug, since
+`setText` does not drive a completer at all: the keys must be typed.
+
 **Do not conclude a key binding is dead from `QTest::keyClick()`.** Whether a symbol needs
 Shift is a layout property, not a Qt one. `Ctrl++` is the shipped `zoom_in` default and is
 exactly what the `+` key emits on an Italian layout, while synthetic input never delivers