diff --git a/solstone/convey/static/app.css b/solstone/convey/static/app.css
index 87988b7bc..1b7fc53cf 100644
--- a/solstone/convey/static/app.css
+++ b/solstone/convey/static/app.css
@@ -161,13 +161,6 @@ body.has-app-bar .workspace {
     margin-left: 0 !important;
   }
 
-  .app-bar {
-    left: 8px;
-    right: 8px;
-    max-width: none;
-    width: auto;
-  }
-
   .menu-backdrop {
     position: fixed;
     inset: 0;
@@ -1897,7 +1890,12 @@ body.menu-all .menu-bar {
 }
 
 @media (max-width: 768px) {
+  /* Mobile: the base .app-bar positioning above re-expands the side inset by
+     source order, so re-assert the mobile inset here, below the base fixed
+     app-bar rule. */
   .app-bar {
+    left: 8px;
+    right: 8px;
     padding: 10px;
     gap: 10px;
   }
diff --git a/tests/test_convey_app_bar.py b/tests/test_convey_app_bar.py
new file mode 100644
index 000000000..b9ce58ff0
--- /dev/null
+++ b/tests/test_convey_app_bar.py
@@ -0,0 +1,115 @@
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright (c) 2026 sol pbc
+
+from __future__ import annotations
+
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+APP_CSS_PATH = REPO_ROOT / "solstone" / "convey" / "static" / "app.css"
+
+
+def _class_specificity(selector: str) -> int:
+    return selector.count(".")
+
+
+def _rule_block(css: str, selector: str, start: int = 0) -> tuple[int, str]:
+    index = css.find(f"{selector} {{", start)
+    assert index != -1, f"{selector} rule was not found"
+    close = css.find("}", index)
+    assert close != -1, f"{selector} rule is not closed"
+    return index, css[index : close + 1]
+
+
+def _block_at(css: str, start: int) -> tuple[int, str]:
+    depth = 0
+    for index in range(start, len(css)):
+        char = css[index]
+        if char == "{":
+            depth += 1
+        elif char == "}":
+            depth -= 1
+            if depth == 0:
+                return start, css[start : index + 1]
+    raise AssertionError(f"block at {start} is not closed")
+
+
+def _media_blocks(css: str, query: str = "@media") -> list[tuple[int, str]]:
+    blocks: list[tuple[int, str]] = []
+    start = css.find(query)
+    while start != -1:
+        blocks.append(_block_at(css, start))
+        start = css.find(query, start + len(query))
+    return blocks
+
+
+def _unconditional_app_bar_geometry_rule(css: str) -> tuple[int, str]:
+    media_spans = [
+        (index, index + len(block)) for index, block in _media_blocks(css, "@media")
+    ]
+    start = css.find(".app-bar {")
+    while start != -1:
+        if not any(
+            span_start <= start < span_end for span_start, span_end in media_spans
+        ):
+            rule_index, rule = _rule_block(css, ".app-bar", start)
+            if "left:" in rule and "right:" in rule and "max-width:" in rule:
+                return rule_index, rule
+        start = css.find(".app-bar {", start + len(".app-bar {"))
+    raise AssertionError("unconditional .app-bar geometry rule was not found")
+
+
+def _mobile_app_bar_left_rules(css: str) -> list[tuple[int, str, str, int]]:
+    rules: list[tuple[int, str, str, int]] = []
+    for block_index, block in _media_blocks(css, "@media (max-width: 768px)"):
+        rule_index = block.find(".app-bar {")
+        if rule_index == -1:
+            continue
+        _, rule = _rule_block(block, ".app-bar", rule_index)
+        if "left:" in rule:
+            rules.append((block_index + rule_index, rule, block, rule_index))
+    return rules
+
+
+def test_desktop_app_bar_geometry_remains_unchanged():
+    css = APP_CSS_PATH.read_text(encoding="utf-8")
+    _, base_rule = _unconditional_app_bar_geometry_rule(css)
+
+    assert "left: var(--menu-bar-width-minimal);" in base_rule
+    assert "right: 0;" in base_rule
+    assert "max-width: 1200px;" in base_rule
+
+
+def test_exactly_one_mobile_app_bar_rule_declares_left_with_comment():
+    css = APP_CSS_PATH.read_text(encoding="utf-8")
+    rules = _mobile_app_bar_left_rules(css)
+
+    assert len(rules) == 1
+
+    _absolute_index, rule, block, rule_index = rules[0]
+    comment_index = block.rfind("/*", 0, rule_index)
+    comment_end = block.find("*/", comment_index)
+
+    assert comment_index != -1
+    assert comment_end != -1
+    assert comment_end < rule_index
+    assert comment_index < rule_index
+    assert "left: 8px;" in rule
+    assert "right: 8px;" in rule
+    assert "padding: 10px;" in rule
+    assert "gap: 10px;" in rule
+
+
+def test_later_mobile_app_bar_rule_wins_by_source_order_at_equal_specificity():
+    css = APP_CSS_PATH.read_text(encoding="utf-8")
+    base_selector = ".app-bar"
+    mobile_selector = ".app-bar"
+    base_index, _base_rule = _unconditional_app_bar_geometry_rule(css)
+    rules = _mobile_app_bar_left_rules(css)
+
+    assert len(rules) == 1
+
+    mobile_index, _mobile_rule, _block, _block_rule_index = rules[0]
+
+    assert _class_specificity(base_selector) == _class_specificity(mobile_selector)
+    assert base_index < mobile_index