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>

app.py

@@ -41,8 +41,14 @@ _NAV_COLLAPSED_WIDTH = 54
 # something if the rail can actually take a width from it, so the expanded rail
 # is a range rather than one number; long project and thread names in RECENTS
 # are the reason someone would widen it.
+#
+# The ceiling is a SHARE of the window, not a pixel count: 360px is a quarter
+# of a 1440 screen and more than a quarter of a 1280 one, where it left the
+# seven Kanban lanes 920px of the 1067 they need. A share behaves the same on
+# every monitor.
 _NAV_MIN_WIDTH = 132
-_NAV_MAX_WIDTH = 360
+_NAV_MAX_SHARE = 0.22
+_NAV_MAX_CEILING = 360
 
 
 def app_icon() -> QIcon:
@@ -220,7 +226,7 @@ class MainWindow(QMainWindow):
         self._nav_wrap = QWidget()
         self._nav_wrap.setObjectName("navWrap")
         self._nav_width = _NAV_EXPANDED_WIDTH   # remembered across collapses
-        self._set_nav_width_range(_NAV_MIN_WIDTH, _NAV_MAX_WIDTH)
+        self._set_nav_width_range(_NAV_MIN_WIDTH, self._nav_max_width())
         nvl = QVBoxLayout(self._nav_wrap)
         nvl.setContentsMargins(0, 0, 0, 0)
         nvl.setSpacing(0)
@@ -372,6 +378,11 @@ class MainWindow(QMainWindow):
 
     def resizeEvent(self, event):  # noqa: N802 - Qt override
         super().resizeEvent(event)
+        # The rail's ceiling is a share of the window, so it moves with the
+        # window. Computed once at construction it was read off a not-yet-sized
+        # window and stuck at 162px on every monitor.
+        if getattr(self, "_nav_wrap", None) is not None and not self._nav_collapsed:
+            self._set_nav_width_range(_NAV_MIN_WIDTH, self._nav_max_width())
         # Keep the floating Help assistant pinned to the bottom-right corner.
         if getattr(self, "help_agent", None) is not None:
             self.help_agent.reposition()
@@ -755,6 +766,11 @@ class MainWindow(QMainWindow):
     def _nav_new_chat_enabled(self) -> bool:
         return bool(self.workspace.project_choices())
 
+    def _nav_max_width(self) -> int:
+        """The rail's ceiling for THIS window, as a share of it."""
+        return max(_NAV_MIN_WIDTH,
+                   min(_NAV_MAX_CEILING, int(self.width() * _NAV_MAX_SHARE)))
+
     def _set_nav_width_range(self, lo: int, hi: int) -> None:
         """setFixedWidth would leave the splitter handle inert — visible, and
         doing nothing when dragged."""
@@ -764,19 +780,19 @@ class MainWindow(QMainWindow):
     def _on_split_moved(self, _pos: int, _index: int) -> None:
         if not self._nav_collapsed:
             self._nav_width = max(_NAV_MIN_WIDTH,
-                                  min(_NAV_MAX_WIDTH, self._nav_wrap.width()))
+                                  min(self._nav_max_width(), self._nav_wrap.width()))
 
     def _toggle_nav(self) -> None:
         if not self._nav_collapsed:
             self._nav_width = max(_NAV_MIN_WIDTH,
-                                  min(_NAV_MAX_WIDTH, self._nav_wrap.width()))
+                                  min(self._nav_max_width(), self._nav_wrap.width()))
         self._nav_collapsed = not self._nav_collapsed
         if self._nav_collapsed:
             width = _NAV_COLLAPSED_WIDTH
             self._set_nav_width_range(width, width)
         else:
             width = self._nav_width
-            self._set_nav_width_range(_NAV_MIN_WIDTH, _NAV_MAX_WIDTH)
+            self._set_nav_width_range(_NAV_MIN_WIDTH, self._nav_max_width())
         self._apply_nav_labels()
         # Same chevron convention as every other collapsible panel: right-
         # pointing (fill-right) means "click to expand", left means "collapse".