Accessibility API Quirks

Building anything on top of the desktop accessibility stack (a screen reader, a test framework, an automation tool, an AI computer-use agent) means meeting three completely different APIs (Linux AT-SPI2 over D-Bus, macOS AXUIElement, Windows UI Automation) and a long tail of toolkit-specific behavior on top of them. The APIs disagree on roles, actions, threading, and even on whether accessibility is on. Most of the surprises are undocumented, or documented only in a vendor’s source tree.

This page is a running catalog of those quirks, collected while building and testing xa11y against real applications across all three platforms. None of them are bugs in xa11y. They are inherent to the platform APIs and toolkits. We publish them here so the next person who hits an empty tree, a phantom error code, or a role that changed between OS releases can find the answer with a search instead of a week of bisecting.

Each entry names the platform, the symptom, the verbatim error or flag where one exists, and a workaround.

Accessibility is opt-in, and often silently off

The single most common failure across every platform: the API connects, the app is found, but the tree is empty or a bare skeleton, with no error to tell you why. Accessibility is opt-in at both the application and the OS-policy level, and the “off” state usually looks identical to “this app has no UI.”

Linux: AT-SPI2 must be enabled on the bus

The accessibility bridge is gated by a D-Bus property, org.a11y.Status.IsEnabled. If it is false (the default on a headless or freshly-booted session), toolkits never publish their trees. Turn it on:

dbus-send --session --dest=org.a11y.Bus /org/a11y/bus \
  org.freedesktop.DBus.Properties.Set \
  string:org.a11y.Status string:IsEnabled variant:boolean:true

You also need the bridge processes actually running:

/usr/libexec/at-spi-bus-launcher --launch-immediately &
/usr/libexec/at-spi2-registryd &

To check whether an app registered with AT-SPI2 at all:

busctl --user tree org.a11y.atspi.Registry | grep -i "<app-name>"

If the app’s subtree is missing, the problem is the app’s accessibility configuration, not your client.

Linux: Chromium and Electron leave the renderer bridge disabled

Chromium-based apps (Google Chrome, Chromium, VS Code, Cursor, Slack, Discord, and every other Electron app) register with AT-SPI2 but expose only an application → frame skeleton until launched with --force-renderer-accessibility. Without it, locator("button").count() returns 0 even though buttons are plainly visible, and the only place this is documented is Chromium’s own source tree.

google-chrome --force-renderer-accessibility
code         --force-renderer-accessibility   # VS Code
cursor       --force-renderer-accessibility

On some Chromium builds the environment variable ACCESSIBILITY_ENABLED=1 has the same effect.

Representative node counts on Ubuntu 24.04 + GNOME 46 (Wayland):

App	Without flag	With flag
VS Code	1	140
Cursor	1	116
Chrome	1	210

You can detect the condition programmatically. Chromium reports Application.ToolkitName == "Chromium", so a Chromium/Electron frame that yields zero filtered children is most likely the renderer bridge being off rather than a bad selector.

Linux: Firefox needs an environment variable

Firefox exposes its tree only when launched with MOZ_ACCESSIBILITY_ATK2=1 set in the environment, the same class of issue as Chromium with a different knob.

MOZ_ACCESSIBILITY_ATK2=1 firefox

Native GTK apps (Nautilus, GNOME Terminal, GNOME Calculator, gnome-text-editor) need no flag, because their AT-SPI2 bridge is enabled by default.

macOS: the client process needs the Accessibility (TCC) permission

macOS refuses to let one process read another’s accessibility tree unless the reading process holds the Accessibility TCC permission (System Settings → Privacy & Security → Accessibility). Without it, queries return an empty tree, not an error.

In CI you grant it by writing the system TCC database and restarting tccd. The critical gotcha: TCC matches the resolved on-disk binary, not a symlink or a launcher. A virtualenv python symlink, or a cargo run wrapper, will not inherit the grant, so you must grant the real interpreter/binary path:

# resolve the real path first
grant_tcc "$(.venv/bin/python -c 'import sys; print(sys.executable)')"

This also bites test runners that use content-addressed binary paths: macOS TCC grants “don’t cope with cargo-hashed test-binary paths”, because every rebuild produces a new hash and a new, ungranted binary.

macOS: web content needs accessibility turned on per-process

Like Chromium on Linux, Chrome/Electron on macOS gate their renderer accessibility tree. Chrome honors --force-renderer-accessibility; WebKit/Safari content is exposed lazily and may require the AXManualAccessibility attribute to be set on the web area before the DOM is reflected into the AX tree.

Roles and the role enum disagree across platforms

macOS roles are open-ended strings, not an enum

AXRole and AXSubrole are plain CFStrings. AXRoleConstants.h lists ~50 conventional roles, but there is no closed enum, and apps and web content report arbitrary values. WebKit alone adds "AXWebArea", "AXLink", "AXHeading", and many more that aren’t in the SDK header. Any code that switches over a fixed set of role strings will silently mishandle real-world content.

Windows: every top-level window is a `Window`, dialogs included

UIA maps all top-level windows, ordinary windows and modal dialogs alike, to UIA_WindowControlTypeId. To tell a dialog apart you must read UIA_IsDialogPropertyId (Windows 10 1703+). Native frameworks such as Qt set IsDialog on a QDialog without populating AriaRole, so the same dialog reports as a dialog on macOS/Linux but as a plain window on Windows unless you check that property.

AriaRole, conversely, is populated only by ARIA/web content (Electron, AccessKit) and never by native Win32/Qt/WPF apps, so you cannot rely on it for native role refinement.

A nasty corollary: because any top-level window with IsDialog=true becomes a dialog, transient system popups (UAC prompts, credential dialogs, Windows Update) will flip a window’s role out from under you. Desktop state is not deterministic.

Windows: `ControlType.Custom` often means “no control type at all”

UIA’s default value for the ControlType property is Custom (50025), so an element whose provider never publishes a control type is indistinguishable from a deliberately custom widget. WinForms hits this constantly: accessible objects that don’t derive from ControlAccessibleObject, a DataGridView’s rows for instance, expose only LegacyIAccessible, so UIA reports Custom even though MSAA still reports ROLE_SYSTEM_ROW.

xa11y therefore reads LegacyIAccessible.Role for Custom elements and maps the MSAA role when it has a clean equivalent. This applies only to Custom: an unrecognized UIA control type stays unknown, so real gaps in the role map remain visible.

Windows: the same grid cell is `DataItem` or `Custom` depending on the framework

There is no single control type for “table cell” in a framework grid. The two Microsoft grids disagree:

WinForms DataGridView. DataGridViewCell.DataGridViewCellAccessibleObject overrides ControlTypePropertyId to DataItem and supports TableItem.
WPF DataGrid. DataGridCellItemAutomationPeer reports AutomationControlType.Custom and supports TableItem + GridItem.

DataItem is also what UIA uses for a row, so neither the control type nor the presence of GridItem identifies a cell on its own. xa11y treats the TableItem pattern as the cell signal for both DataItem and Custom elements, and falls back to a structural check (a DataItem whose parent is a DataItem) for bridges that expose no table patterns at all, such as AccessKit’s.

Windows: WinForms range controls have no `RangeValue` pattern

A WinForms TrackBar reports ControlType.Slider and a NumericUpDown reports ControlType.Spinner, but neither supports RangeValuePattern, and TrackBar.TrackBarAccessibleObject advertises Value and LegacyIAccessible only, and UpDownBase.UpDownBaseAccessibleObject inherits the Invoke-only base set. The current value is readable as a string through ValuePattern; the minimum and maximum are not exposed at all. So a control that looks like a slider in the tree can still have no numeric value and no range, and stepping it through the accessibility API is not possible.

WPF is the counter-example on the same platform: SliderAutomationPeer and ProgressBarAutomationPeer both implement IRangeValueProvider, so identical control types carry a value and a range there. “Is this a slider?” and “can I read its value?” are separate questions on Windows.

Linux: roles are a 131-value C enum on the wire

AtspiRole is a C enum (131 values) sent as a u32. Examples: PUSH_BUTTON (43), CHECK_BOX (7), ENTRY (79), TEXT (61), FRAME (23). States are a separate 64-bit bitfield (AtspiStateType) rather than properties: ENABLED (8), FOCUSED (12), VISIBLE (30), EDITABLE (7), CHECKED (4).

Qt tabs: `tab` on Windows, `radio_button` on macOS

Qt’s accessibility bridges map a QTabBar tab to different roles per platform: Windows UIA reports tab (inside a tab_group), while the macOS AX bridge reports radio_button. A selector that should work on both platforms has to match either:

dialog.descendant("tab[name='Settings'], radio_button[name='Settings']").press()

WebKitGTK changed the role it reports for `<textarea>` between releases

WebKitGTK 2.52 changed which AT-SPI role it reports for an HTML <textarea>, breaking any selector keyed on the old role after a routine libwebkit2gtk-4.1 upgrade (2.50.4 → 2.52.3). The portable fix is to never trust the toolkit’s text role directly. Derive single-line vs multi-line from the MULTI_LINE state bit, which is consistent across GTK (GtkEntry/GtkTextView), Qt (QLineEdit/QPlainTextEdit), and every WebKit version.

One process, several “applications”

Windows: Qt registers dialogs as separate UIA apps

A QDialog opened inside a host process (e.g. a Qt submitter dialog inside a C++ DCC application) can register with UIA as its own top-level “application”, sharing the host’s PID but living outside the host app’s element tree. Searching the host app for the dialog finds nothing, and you must attach to the dialog’s own app. Predicate-based discovery handles this without hand-rolling an enumeration loop:

dialog_app = xa11y.App.find(
    lambda a: a.pid == proc.pid and a.name.startswith("My Submitter"),
    timeout=60.0,
)

On macOS AX the same dialog is an ordinary child window of the host app, and so dialog discovery is one of the few places where platform-specific code is legitimately required.

Both of the following are confirmed against live trees:

The dialog’s accessible name is the QApplication display name, not windowTitle(). Both the UIA “app” and the dialog window inside it surface QGuiApplication::applicationDisplayName() (typically the app name + version), so match on a name prefix rather than the window title you see in the title bar.
Child popups may be hosted elsewhere. A QMessageBox shown from such a dialog can appear in the host application’s tree, not the dialog’s UIA app. When the owner is unpredictable, search from the system root (module-level xa11y.locator(...)) and wait for the popup’s body text.

Qt forms don’t label their fields

QFormLayout does not propagate a row’s label to the field’s accessible name. In practice sibling widgets surface with the enclosing group’s name (e.g. three spin boxes all named "Job Properties") or with an empty name. Selectors can’t disambiguate by name alone, so pin the widget with .nth(n) (1-based, tree order) or scope to a parent group first, and re-verify the indices when the form’s layout changes.

Action names and mechanisms are not standardized

Linux: AT-SPI action names differ by toolkit

Actions are string-named via org.a11y.atspi.Action, and the names are not standardized. GTK calls its default-activate action "click"; Qt calls it "Press". Chromium uses its own: "doDefault" (default activation, ~190 elements in a single Chrome window) and "showContextMenu" (~230 elements). A client that looks for a hardcoded "press" will report ActionNotSupported on half the ecosystem unless it normalizes through an alias table.

Chromium on Linux exposes no `EditableText` interface, anywhere

This is a big one for text input. No Chromium element provides org.a11y.atspi.EditableText anywhere in its tree (confirmed across 500+ nodes). There is no accessibility path to set the value of a Chromium/Electron text field on Linux, and attempts hit D-Bus UnknownMethod / UnknownInterface errors. The only options are keyboard synthesis (xdotool / ydotool / uinput) or a fix upstream in Chromium.

GtkMenuButton, AdwMenuButton, and AdwSplitButton present an outer push button accessible that advertises NActions = 0, so pressing it raises ActionNotSupported. It wraps an inner toggle button that carries the real click action. Every stock GNOME app has them (Calculator, Text Editor, Characters, Baobab, Logs, Clocks), so “the button I can see has no press action” is a common, confusing result. The workaround is to walk a bounded slice of the outer widget’s subtree (gated on Application.ToolkitName == "GTK") and invoke the single matching descendant.

GTK4 `CheckButton` reports `NActions = 0`

A GTK4 CheckButton can report zero actions via the Action interface even though it is toggleable, so toggle() via DoAction is unavailable and toggling must go through the press path. AccessKit’s AT-SPI bridge has the mirror-image quirk: it hardcodes the action name as "click", never "toggle".

macOS has no `AXToggle`, so `AXPress` is the native toggle

There is no AXToggle action. AXPress is the platform’s toggle for checkboxes, switches, and radio buttons. Code expecting a dedicated toggle action on macOS will never find one.

macOS has no `ScrollIntoView` equivalent

There is no AX action to scroll an element into view. On Linux this maps to Component.ScrollTo; on Windows to ScrollItemPattern.ScrollIntoView; on macOS it really is a no-op.

Numeric vs text values diverge

Linux: the org.a11y.atspi.Value interface is f64-only. Text values require EditableText. If neither is present you get a “not supported” result. WebKit sliders on Linux historically did not honor Value.SetCurrentValue.
macOS: AXValue is set directly for both text and numeric.
Windows: numeric goes through RangeValuePattern, text through ValuePattern, and type_text is a non-atomic read-modify-write splice via TextPattern.GetSelection() + ValuePattern.SetValue().

Selection is reported through a different channel on every platform

Windows: SelectionItemPattern.IsSelected where the framework provides it, but frameworks that predate the pattern publish selection only as the MSAA STATE_SYSTEM_SELECTED bit on LegacyIAccessible.State. WinForms grid cells are the common case: their pattern set is Legacy/Invoke/Value/TableItem/GridItem and no SelectionItem. xa11y reads both.
macOS: per-element AXSelected where it exists; Qt’s Cocoa bridge provides no per-cell AXSelected at all and exposes the selection only through the container’s AXSelectedChildren, so xa11y derives cell selection from the table.
Linux: the AT-SPI SELECTED state bit, set per element.

Qt spin boxes step rather than set

Setting a Qt QSpinBox / QDoubleSpinBox value through the accessibility API does not work on either Windows or macOS: depending on the platform and call, the set either raises or reports success without changing the value. The reliable cross-platform pattern is to step with increment() / decrement() and read the value back after each step. Don’t trust a fixed step count, because a single increment() can occasionally advance the value by more than one:

spin = dialog.descendant("spin_button[name='Priority']")
current = int(spin.element().value)
while current != target:
    spin.increment() if current < target else spin.decrement()
    current = int(spin.element().value)

(Bound the loop in real code so an out-of-range target can’t spin forever.) Text fields and checkboxes are unaffected, and set_value() and toggle() work on Qt as expected.

Element references go stale, constantly

All three backends mint a fresh handle on every read

Windows/UIA, macOS/AX, and Linux/AT-SPI2 each mint a brand-new handle every time you cache or re-read an element. You cannot use a handle’s identity to dedup or compare elements across two queries, because the same on-screen element returns disjoint handles, so naive identity-based dedup silently returns zero results. Key elements by a stable property instead: UIA GetRuntimeId, AT-SPI (bus_name, object_path), or a structural tree path.

AccessKit republishes the whole subtree on any mutation

AccessKit’s AT-SPI bridge regenerates the entire subtree (and all accessible object paths) on a structural mutation. A snapshot captured before an action is invalidated the moment that action rebuilds the tree, so a second operation on the old reference surfaces a stale-object platform error. The dependable pattern is to re-resolve the selector after every mutation rather than holding element handles.

UIA tree updates are asynchronous

After an action that changes structure (closing a dialog, moving to another view), the UIA tree can take longer than 100 ms to settle. During that window a perfectly valid selector is transiently un-findable:

SelectorNotMatchedError: No element matched: button[name="OK"]

Poll/retry around structural changes rather than asserting immediately.

Phantom errors that aren’t really errors

Windows: `EVENT_E_ALL_SUBSCRIBERS_FAILED (0x80040201)`

Qt’s UIA providers for QTabBar/QTabWidget raise automation events synchronously inside their Invoke() / Select() handlers. When no UIA event subscriber is active (typical on a CI runner with no screen reader), COM returns:

EVENT_E_ALL_SUBSCRIBERS_FAILED (0x80040201)

Qt propagates this back through the action call even though the UI action itself completed successfully. The same code can surface from FindAllBuildCache during a find. xa11y treats 0x80040201 as success on action calls (press/toggle/select). On query calls, where a value is still needed, it retries the call a few times before giving up.

Windows: `FindAllBuildCache` silently drops fragment elements

FindAllBuildCache(TreeScope_Subtree) rooted at a UIA fragment returns incomplete results, because the default Control View filter excludes any element that doesn’t declare IsControlElement=true. Virtual/fragment providers (Qt, AccessKit) often don’t, so they vanish. Pass a TrueCondition tree filter (Raw view) to see them.

Windows: `SetFocus` fails on disabled elements

UIA SetFocus fails outright on a disabled element. Tests and automation should target an always-enabled element to verify focus behavior.

Windows: screenshots fail in Session 0 / disconnected RDP

GDI BitBlt returns ERROR_INVALID_HANDLE in disconnected-RDP or Session-0 contexts, because there is no visible desktop to capture.

Threading and process model

Windows UIA is COM, MTA-only. Event handlers fire on a background MTA thread (CoInitializeEx(NULL, COINIT_MULTITHREADED)). Use per-handler RemoveXxxEventHandler, since RemoveAllEventHandlers nukes every concurrent subscription in the process.
macOS AXObserver is CFRunLoop-bound. Callbacks deliver a live AXUIElementRef valid only during the callback, so capture what you need synchronously.
Linux AT-SPI is D-Bus. A dedicated accessibility bus (separate from the session bus) handles every message. The registry daemon tracks apps and routes events.

Memory safety: AX values can throw through your FFI

On macOS, a misbehaving AX value’s -release or -getTypeID can throw an NSException. If it unwinds through an extern "C" boundary it aborts the process. Worse, partial extraction paths can hit use-after-free / double-release (e.g. releasing a CFNumber and then reading its type-id). Any production AX client must wrap raw CoreFoundation / AX calls (CFRetain, CFRelease, CFGetTypeID, CFNumberGetValue, CFArrayGetValueAtIndex, …) in @try/@catch and treat every FFI return as fallible.

Events: every platform has gaps

Push-based accessibility events are richer than polling, but each platform omits things the others deliver:

Linux AT-SPI2 has no menu-open/close signal at all. MenuOpened / MenuClosed can never fire. Consumers must poll or watch structural changes.
GTK4 sometimes skips Focus:Focus, emitting only Object:StateChanged(focused, true). Listen for both.
StateChanged{Enabled} arrives as either StateChanged(enabled, _) or StateChanged(sensitive, _), since AT-SPI2 still exposes both names.
WebKit2GTK and Electron often omit Text:TextChanged, so you may have to fall back to ValueChanged on text roles.
macOS AXValueChanged carries no position or delta, only the element. There is no AXTextChanged-with-position notification; diffing old vs new text is the only option.
macOS scopes many notifications to element-level observers: table row/selection changes, AXMenuItemSelected, AXCreated/AXMoved/AXResized, AXLayoutChanged are not delivered to an app-level observer.
Windows UIA has no first-class window-activated event. Inferring it from focus changes is lossy: it misses alt-tab and tool windows, and fires spuriously on in-app focus moves. Older/non-native apps (Java Swing, VB6, Adobe products, some browsers) have incomplete UIA providers and may not fire events at all.

Performance: reading a tree is O(N) cross-process calls

Two of the three platforms have no bulk-read API, so traversing a tree means a storm of IPC:

Platform	Per-node cost	Bulk subtree fetch?
Linux (AT-SPI2)	6-10 D-Bus round-trips per node	No. `GetChildren` returns references, and you re-query each child. (`Cache.GetItems` exists but returns only a partial property set for the whole app.)
macOS (AX)	2-3 Mach IPC calls per node	No, but `AXUIElementCopyMultipleAttributeValues` batches one node’s attributes into a single round-trip.
Windows (UIA)	One call for a whole subtree	Yes. `IUIAutomationCacheRequest` + `FindAllBuildCache` fetches an entire subtree with chosen properties/patterns in one cross-process call.

On Linux and macOS, a naive full-tree walk of a complex app is slow enough to feel hung. Prefer narrow, combinator-constrained queries (toolbar > button[name='Save']) so the engine never descends subtrees it doesn’t need.

Text and string surprises

Bidi control characters leak into reported strings

Platforms embed Unicode bidirectional format controls into the strings they report (name, value, description); macOS does it for some LTR app configurations, and RTL apps do it everywhere. These are invisible but break naive equality:

el.value == "5"   # False — the value is actually "⁦" + "5" + "⁩"

The offenders are U+200E (LRM), U+200F (RLM), U+202A..U+202E (embeddings/overrides), and U+2066..U+2069 (isolates). Strip them before comparing; keep ZWJ/ZWNJ, which are legitimate content.

at-spi2-core can drop names containing control characters

Some at-spi2-core configurations drop an accessible’s Name entirely when it contains non-printable control characters, so an element with a perfectly good visible label becomes undiscoverable by name. This is configuration-dependent (the same app can keep the name under a different at-spi2-core build), which makes it especially confusing.

Linux input simulation and Wayland

Wayland has no standard accessibility-driven input path, and the architecturally “correct” route is not broadly available yet:

xdg-desktop-portal.RemoteDesktop (libei) is implemented today only by xdg-desktop-portal-gnome and -kde, and needs a GDM/SDDM-managed session, so there is no working portal backend on wlroots compositors (sway, Hyprland) or headless servers.
/dev/uinput is the compositor-agnostic fallback (the same mechanism xdotool --using-uinput, ydotool, wtype, Steam Input, and Wine use), but it requires input group membership, which grants global input read/write, comparable to macOS Input Monitoring:
Terminal window
```
sudo usermod -aG input $USER   # then re-login
```

In containers, Docker’s /dev tmpfs isolation hides host-created /dev/input/event* nodes even with -v /dev/input:/dev/input; a udev rule like KERNEL=="event[0-9]*", MODE="0666" is needed to expose them.

macOS focus and launch races

CGEventPost (and OS focus) target whichever app is frontmost. On a CI runner an onboarding/background window (Setup Assistant, Notification Center, Software Update) can hold the front slot and silently swallow every synthetic event, leaving an empty event log. Claim frontmost for your target app before driving input. A window can be visible without having focus.
open -a has a -600 quit-then-relaunch race. Re-launching an app that is mid-quit fails with error -600. “The process exists” also does not mean “its accessibility tree is ready”, so poll for the tree, not just the PID.

Coordinate systems

macOS reports points, not pixels. On a Retina display 1 point = 2 physical pixels, so hit-testing against a screenshot taken in pixels will be off by the scale factor.
Windows and Linux report logical (device-independent) coordinates, with Screenshot.scale bridging to physical pixels. On Linux the scale is exact where reliably detectable and falls back to 1.0 (coordinates then physical) otherwise. See platform details. Multi-monitor setups produce negative coordinates for displays positioned left of or above the primary.