Pin rail row icons to the left edge instead of letting Qt centre them

QStyledItemDelegate hands the style decorationAlignment = AlignHCenter |
AlignVCenter. For a row with a label that is invisible — the icon sits left of
the text either way — but every rail row loses its label when the rail
collapses to 54px, and a centred decoration then lands wherever the column
happens to leave room. _NavItemDelegate overrides it to AlignLeft, so the icon
is on the row's left padding regardless of column width, style or platform.

Honest limit: this was reported from the running app and I could not reproduce
it offscreen — sweeping the column width leaves the icon at a fixed x with or
without the delegate. What is verifiable is the instruction, and centring is
the only mechanism by which Qt can put a label-less row's icon anywhere but
the left. check_rail_align now asserts the flag itself, which fails (132) the
moment the delegate is removed, alongside the column sweep.

Settings follows the rows to their new inset (padding-left 7px → 4px). Its
label still sat 3px left of Dashboard's because a button leaves less air
between icon and label than a tree row does; a 19px icon box closes that to
1px, with the 16px glyph left-aligned inside it.

15/15 checkers pass, no flakes this run.

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

tools/check_rail_align.py

@@ -139,6 +139,7 @@ def main() -> int:
         win._toggle_nav()
         app.processEvents()
         fails += compare(theme_name, rail, opened, closed)
+        fails += column_widths_do_not_move_icons(win, app, theme_name)
 
     print()
     for f in fails:
@@ -149,6 +150,63 @@ def main() -> int:
     os._exit(1 if fails else 0)
 
 
+def column_widths_do_not_move_icons(win, app, theme_name):
+    """The icon must not care how wide the column is.
+
+    On the machine that reported this the column matched the rail and the icons
+    sat in the middle; in a test render the column stayed wider than the view
+    and the same code drew them at the left. So sweep the width and require the
+    icon to hold still.
+    """
+    from PySide6.QtWidgets import QStyle, QStyleOptionViewItem
+
+    if not win._nav_collapsed:
+        win._toggle_nav()
+        app.processEvents()
+    fails, seen = [], {}
+    # The invariant that matters. A centred decoration is the only way Qt can
+    # put a label-less row's icon anywhere but the left edge, and how far it
+    # travels depends on the box the column hands it — which is why this
+    # reproduces on one machine and not another. Require the instruction
+    # itself, not just the pixel it happens to produce here.
+    from PySide6.QtCore import Qt as _Qt
+    for tree_name in ("nav", "nav_bottom"):
+        tree = getattr(win, tree_name)
+        index = tree.indexFromItem(tree.topLevelItem(0), 0)
+        opt = QStyleOptionViewItem()
+        tree.initViewItemOption(opt)
+        tree.itemDelegate().initStyleOption(opt, index)
+        align = int(opt.decorationAlignment)
+        if align & int(_Qt.AlignHCenter) or not align & int(_Qt.AlignLeft):
+            fails.append(f"{theme_name} {tree_name}: decorationAlignment={align}"
+                         f" — icon is free to drift off the left edge")
+    for tree_name in ("nav", "nav_bottom"):
+        tree = getattr(win, tree_name)
+        keep = tree.columnWidth(0)
+        for width in (tree.viewport().width(), 70, 100, 140):
+            tree.setColumnWidth(0, width)
+            app.processEvents()
+            index = tree.indexFromItem(tree.topLevelItem(0), 0)
+            opt = QStyleOptionViewItem()
+            tree.initViewItemOption(opt)
+            opt.rect = tree.visualRect(index)
+            tree.itemDelegate().initStyleOption(opt, index)
+            x = tree.style().subElementRect(
+                QStyle.SE_ItemViewItemDecoration, opt, tree).left()
+            seen.setdefault(tree_name, []).append((width, x))
+        tree.setColumnWidth(0, keep)
+        app.processEvents()
+        xs = {x for _w, x in seen[tree_name]}
+        if len(xs) > 1:
+            fails.append(f"{theme_name} {tree_name}: icon x changes with the "
+                         f"column width — {seen[tree_name]}")
+    print(f"  column sweep: " + "  ".join(
+        f"{n}={[x for _w, x in v]}" for n, v in seen.items()))
+    win._toggle_nav()
+    app.processEvents()
+    return fails
+
+
 def compare(theme_name, rail, opened, closed):
     print()
     print(f"theme={theme_name}")