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+# Windows Accessibility Bridge
+
+This code provides a bridge between our internal Accessibility
+interfaces (implemented on all visible 'things' in the suite: eg.
+windows, buttons, entry boxes etc.) - and the Windows `MSAA` /
+`IAccessible2` COM interfaces that are familiar to windows users and
+Accessible Technologies (ATs) such as the NVDA screen reader.
+
+The code breaks into three bits:
+
++ `source/service/`
+	+ the UNO service providing the accessibility bridge.
+	  It essentially listens to events from the LibreOffice
+	  core and creates and synchronises COM peers for our
+	  internal accessibility objects when events arrive.
+
++ `source/UAccCom/`
+	+ COM implementations of the `MSAA` / `IAccessible2` interfaces
+	  to provide native peers for the accessibility code.
+
++ `source/UAccCOMIDL/`
+	+ COM Interface Definition Language (IDL) for UAccCom.
+
+Here is one way of visualising the code / control flow
+
+    VCL <-> UNO toolkit <-> UNO a11y <-> win a11y <-> COM / IAccessible2
+
+    vcl/ <-> toolkit/ <-> accessibility/ <-> winaccessibility/ <-> UAccCom/
+
+## Threading
+
+It's possible that the UNO components are called from threads other
+than the main thread, so they have to be synchronized. It would be nice
+to put the component into a UNO apartment (and the COM components into STA)
+but UNO would spawn a new thread for it so it's not possible.
+The COM components also call into the same global `AccObjectWinManager`
+as the UNO components do so both have to be synchronized in the same way.
+
+So we use the `SolarMutex` for all synchronization since anything else
+would be rather difficult to make work.  Unfortunately there is a
+pre-existing problem in vcl with Win32 Window creation and destruction
+on non-main threads where a synchronous `SendMessage` is used while
+the `SolarMutex` is locked that can cause deadlocks if the main thread is
+waiting on the `SolarMutex` itself at that time and thus not handing the
+Win32 message; this is easy to trigger with `JunitTests` but hopefully
+not by actual end users.
+
+## Debugging / Playing with winaccessibility
+
+If NVDA is running when soffice starts, IA2 should be automatically enabled
+and work as expected. In order to use 'accprobe' to debug
+it is necessary to override the check for whether an AT (like NVDA) is
+running; to do that use:
+
+    SAL_FORCE_IACCESSIBLE2=1 soffice.exe -writer
+
+Then you can use accprobe to introspect the accessibility hierarchy
+remotely, checkout:
+
+<http://accessibility.linuxfoundation.org/a11yweb/util/accprobe/>
+
+But often it's more useful to look at NVDA's text output window.