Add readme for NVDA Helper

- Also fix a typo in #10552

nvdaHelper/readme.md

@@ -0,0 +1,54 @@
+## NVDA Helper
+
+Parts of NVDA Helper are used to capture and cache information, in-process, within web browsers and other applications.
+This cache is called a virtual buffer.
+While virtual buffers apply to several different application types, it may first be easiest to think about how they
+work with browsers, the rest of this document will take a web browser centric view unless specified otherwise.
+
+### Configuring Visual Studio
+The following steps won't prepare a buildable solution, but it will enable intellisense.
+You should still build on the command line to verify errors.
+
+- Ensure you have built NVDA on the command line first.
+- Create a new project from existing code
+- Type: Visual C++
+- Set the `<repo root>/nvdaHelper/` directory as the project file location.
+- Project name: "nvdaHelper"
+- Add files to the project from these folders: checked.
+- Other defaults are fine
+- Includes: `../include;../miscdeps/include;./;../build\x86_64;%(AdditionalIncludeDirectories)`
+- Defines: `WIN32;_WINDOWS;_USRDLL;NVDAHELPER_EXPORTS;UNICODE;_CRT_SECURE_NO_DEPRECATE;LOGLEVEL;_WIN32_WINNT;_WIN32_WINNT_WIN7;`
+- Force Includes: `winuser.h`
+- Create the project.
+- Open the project settings and change the following:
+  - General -> Windows SDK: 10.0.17763.0 (or whatever is the latest on your system)
+  - C/C++ -> Language -> C++ Language Standard -> ISO C++14 Standard (/std:c++14)
+
+### Virtual Buffer Backends
+
+This code runs within the target applications process, it is responsible for building the virtual buffer.
+The base classes are in the `storage.h` and `storage.cpp`.
+The `storage` classes are implementation agnostic, they don't use the accessibility APIs directly and thus don't have
+the same performance concerns as the code that directly interacts with the accessibility APIs,
+such as the code in `gecko_ia2.cpp`.
+A buffer (`VBufStorage_buffer_t`) has many `Nodes` that make up the document.
+The `VBufStorage_buffer_t` class owns the *Node instances, such as `controlFieldNode` and `textFieldNode`.
+The buffer (`VBufStorage_buffer_t`) is responsible for creating (in `fillVBuf`) and caching the nodes that make up the
+document.
+An element in a HTML document becomes a `controlFieldNode`.
+The inner text of a HTML element would become a `textFieldNode`.
+Extra information that we want to be able to present to the user is collected as these nodes are created.
+For instance the name, description, role, and many other pieces of information.
+A Node has a pointer to its parent node, child nodes, and sibling nodes, we have a graph of the virtual buffer.
+Each `controlFieldNode` (like an element, remember?) has an identifier (`VBufStorage_controlFieldNodeIdentifier_t`).
+Given an identifier, a `controlFieldNode` can be looked up using `controlFieldNodesByIdentifier`.
+Though, take care, looking up nodes is only possible after that part of the tree has been rendered.
+The `controlFieldNode`s are added to the buffer via `addControlFieldNode`, which also adds these nodes to the map.
+
+### Gecko IA2
+
+This is a virtual buffer backend implementation used for Firefox and Chrome.
+When `fillVBuf` is called, it recursively descends through the child elements that we care about, creating either
+`controlFieldNode`s or `textFieldNode`s for them.
+This code is responsible for interacting with the IA2 accessibility API, these calls should be minimised for
+ performance reasons.

nvdaHelper/vbufBackends/gecko_ia2/gecko_ia2.cpp

@@ -1158,8 +1158,8 @@ VBufStorage_fieldNode_t* GeckoVBufBackend_t::fillVBuf(
 	if (labelId) {
 		auto labelControlFieldNode = buffer->getControlFieldNodeWithIdentifier(docHandle, labelId.value());
 		if (labelControlFieldNode) {
-			bool isDecendant = buffer->isDescendantNode(parentNode, labelControlFieldNode);
-			if (isDecendant) {
+			bool isDescendant = buffer->isDescendantNode(parentNode, labelControlFieldNode);
+			if (isDescendant) {
 				parentNode->addAttribute(L"labelledByContent", L"true");
 			}
 		}