Size the board and the rail by proportion, not by pixel constants

"Some screens scroll sideways, some don't" had two causes, and both were a
number that was right on one monitor.

The lanes. _KanbanColumn set a 150px floor with a comment claiming seven of
them fit a 1280 window; measured, the row wanted 1242px where 1280 leaves 1091,
so a 1280 screen scrolled and a 1920 one did not — this machine has one of each.
The floor is gone. The seven lanes share the board through the layout's
stretch, which is a proportion of whatever width there is, and the only pixel
question left — how narrow before scrolling beats squeezing — is answered from
the font: eight characters of a title plus padding, so it holds at 125%/150%
scaling too. No horizontal scroll now at 1280×720 through 1936×1048, at three
rail widths each, including the rail dragged to its maximum.

Each lane also grew its own scrollbar: QListWidget's column hint runs 1-6px past
its viewport, and which lanes overflowed changed with the window width — 36 of
38 widths had at least one. Cards word-wrap, so there was never anything to
reach sideways; the bar is off.

The rail. Its 360px ceiling was a quarter of a 1440 screen and more than that of
a 1280 one. It is now 22% of the window, capped at 360 — and recomputed on
resize, which is its own bug fixed: read once at construction off a not-yet-sized
window, it sat at 162px on every monitor, so the handle barely moved.

check_kanban_scroll covers five screen sizes × three rail widths. A rail dragged
to maximum on a 1280 screen still scrolls the board — that is the user's own
trade, so it is reported rather than failed.

19/19 checkers pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/schedule_task_tab.py

@@ -53,11 +53,18 @@ class _KanbanColumn(QListWidget):
         # → "Delete N selected" to bulk-remove tasks instead of one at a time.
         self.setSelectionMode(QAbstractItemView.ExtendedSelection)
         self.setWordWrap(True)
-        # Narrow enough that all SEVEN lanes fit on one screen, which is what
-        # the design asks for — at 190 the seventh (Paused) fell off the right
-        # edge and needed a horizontal scroll to reach.
-        # 7 × 150 + 6 gaps = 1098px, inside the content area of a 1280 window.
-        self.setMinimumWidth(150)
+        # Cards wrap, so there is never anything to reach by scrolling sideways
+        # — but QListWidget's own column hint runs 1-6px past the viewport, and
+        # a lane sprouted a horizontal scrollbar at 36 of 38 window widths I
+        # measured. Which lanes grew one changed with the width, which is why it
+        # looked like it depended on the screen.
+        self.setHorizontalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
+        self.setResizeMode(QListWidget.Adjust)   # re-wrap on every resize
+        # No pixel floor here. A fixed one is always wrong on some screen:
+        # 190 lost the seventh lane, 150 still wanted 1242px where a 1280
+        # window leaves 1091 — so the 1280 monitor scrolled sideways and the
+        # 1920 one did not, same app, same build. The board divides whatever
+        # width it has by seven instead; see _fit_lanes().
 
     def dropEvent(self, event):  # noqa: N802
         source = event.source()
@@ -146,6 +153,9 @@ class ScheduleTaskTab(QWidget):
             cols.addWidget(holder)
             self.columns[status] = col
             self.column_headers[status] = head
+        self._board_scroll = scroll
+        self._board_gap = cols.spacing()
+        scroll.viewport().installEventFilter(self)
         self._view_stack.addWidget(scroll)
         self.calendar = CalendarView()
         self.calendar.edit_task.connect(self._edit_task)
@@ -198,6 +208,30 @@ class ScheduleTaskTab(QWidget):
             self._save_and_refresh(dlg.edited_task)
             self.status_message.emit(tr("schedtask.msg_created"))
 
+    # ---- lane widths ------------------------------------------------------
+    #
+    # The seven lanes share the board equally — that is the layout's stretch
+    # doing the work, so the split is a proportion of whatever width there is,
+    # on any monitor. The only pixel question left is how narrow a lane may get
+    # before scrolling sideways beats squeezing, and that is a question about
+    # TEXT: roughly eight characters of a task title plus its padding. Reading
+    # it off the font keeps it right at 125%/150% scaling and at a user's own
+    # font size, where a constant would not be.
+    _LANE_FLOOR_CH = 8
+
+    def eventFilter(self, obj, event):  # noqa: N802
+        from PySide6.QtCore import QEvent
+
+        if obj is self._board_scroll.viewport() and event.type() == QEvent.Resize:
+            self._fit_lanes()
+        return super().eventFilter(obj, event)
+
+    def _fit_lanes(self) -> None:
+        floor = self.fontMetrics().averageCharWidth() * self._LANE_FLOOR_CH + 24
+        for col in self.columns.values():
+            if col.minimumWidth() != floor:
+                col.setMinimumWidth(floor)
+
     # ---- board rendering ---------------------------------------------------
     def _card_text(self, t: dict) -> str:
         prio = _PRIORITY_ICONS.get(t.get("priority", "medium"), "")