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>

tools/check_rail_resize.py

@@ -38,7 +38,7 @@ def main() -> int:
     seed()
 
     from cowork_local.app import (
-        _NAV_COLLAPSED_WIDTH, _NAV_MAX_WIDTH, _NAV_MIN_WIDTH, MainWindow)
+        _NAV_COLLAPSED_WIDTH, _NAV_MIN_WIDTH, MainWindow)
     from cowork_local.config import AppConfig
     from cowork_local.i18n import set_language
     from cowork_local.state import AppContext
@@ -72,10 +72,12 @@ def main() -> int:
     if narrow >= wide:
         fails.append(f"keo hep lai khong an: {narrow}px")
 
-    over = drag_to(_NAV_MAX_WIDTH + 200)
-    print(f"keo qua max: {over}px (tran {_NAV_MAX_WIDTH})")
-    if over > _NAV_MAX_WIDTH:
-        fails.append(f"rail vuot tran: {over} > {_NAV_MAX_WIDTH}")
+    # The ceiling is a share of the window now, so ask the window for it.
+    ceiling = win._nav_max_width()
+    over = drag_to(ceiling + 200)
+    print(f"keo qua max: {over}px (tran {ceiling} = {ceiling * 100 // win.width()}% cua so)")
+    if over > ceiling:
+        fails.append(f"rail vuot tran: {over} > {ceiling}")
 
     under = drag_to(20)
     print(f"keo duoi min: {under}px (san {_NAV_MIN_WIDTH})")
@@ -83,7 +85,7 @@ def main() -> int:
         fails.append(f"rail thap hon san: {under} < {_NAV_MIN_WIDTH}")
 
     # a dragged width has to come back after a fold
-    chosen = drag_to(280)
+    chosen = drag_to(min(280, win._nav_max_width()))
     win._toggle_nav()
     app.processEvents()
     folded = rail.width()
@@ -97,6 +99,22 @@ def main() -> int:
     if abs(back - chosen) > 4:
         fails.append(f"mo lai quen be rong da keo: {back} thay vi {chosen}")
 
+    # The ceiling is a share, so it has to move with the window — it was read
+    # once at construction and stuck at 162px on every monitor.
+    seen = {}
+    for w in (1280, 1600, 1936):
+        win.resize(w, 900)
+        app.processEvents()
+        split.setSizes([2000, 1])       # drag the handle as far right as it goes
+        app.processEvents()
+        seen[w] = (rail.width(), win._nav_max_width())
+        print(f"cua so {w}: keo het co -> {seen[w][0]}px (tran {seen[w][1]}px)")
+    for w, (got, ceiling) in seen.items():
+        if abs(got - ceiling) > 4:
+            fails.append(f"cua so {w}: keo het chi duoc {got}px, tran la {ceiling}px")
+    if len({c for _g, c in seen.values()}) == 1:
+        fails.append("tran khong doi theo be rong cua so — dang la px co dinh")
+
     print()
     for f in fails:
         print("FAIL  " + f)