* `buffers` is an unnecessary intermediate variable. Any text buffer present in `buffersToTag`
  but not also present in `buffers` will end up ignored by `ComputeNewTagTree`, and thus not
  influence the result of `ConvertToTagTrees`. Based on this, `buffers` was eliminated to
  simplify the implementation.
* When `spansTagged` has only a single span, it's possible that other tag trees in `oldTagTrees`
  need to propagate forward. The early return failed to account for this, and if hit would drop
  `oldTagTrees` tags for buffers not equal to the buffer from `spansTagged`. In addition, the
  early return failed to account for the case where `spansTagged` only contained a single
  span from a buffer not requested per `buffersToTag`. Following the elimination of `buffers`,
  the value of a special case handling here was notably reduced and the edge case was simply
  eliminated in favor of the remaining code in the method.
* The patter of Select followed by ToLookup was replaced by a single call to ToLookup specifying
  both the keys and the values of the resulting lookup.

Merge dev15.8-preview3 to master

* move off UI thread in option changed event handler

* added comment.

* updated type name and comment

Test stability fixes

* Fixed 26688

* Fixed 26743

* Document breaking change.

* Fix build break

* PR Comments

Shut down the dispatcher after WpfFact and WpfTheory tests

* move some GetServices from constructor into CreateEditorInstance()

* PR comments

* remove Package param from constructor

Replace a hard crash with an NFW

* Copy escape scope of pattern locals from source expressions

* Document breaking change

* Join the STA thread before completing the test (with a hang mitigating timeout)
* Update the STA thread name to reflect the current test method

* Reporting errors on foreach ref/readonly declarations in async/iterator methods

* PR Comments

* ToBadExpression() checking incorrect HasAnyErrors API

* Undo fix for 15.8

* Fix build break

* Make sure to dispose of staThreadStartedEvent
* Update documentation regarding Dispatcher.InvokeShutdown

Make ShadowCopyAnalyzerAssemblyLoader threadsafe

This reverts commit 3b534b39.

Fixes Watson bug VSO [400829](https://devdiv.visualstudio.com/DevDiv/NET%20Developer%20Experience%20Productivity/_workitems/edit/400829)

* workaround on long pending incremental parsing requests

* use GetSyntaxTreeAsync so that underlying AsyncLazy reuse same Task for parsing when there are multiple requests

* comment updated

* more comments

Fixes #20169

Trap all exceptions from the IVsFileChange service

Merge dev15.8-preview3 to master

Add an async test case for a try/finally with an uncaught exception

Previously we only trapped some exceptions from the VS file change
service, letting the rest propagate. It turns out this doesn't work
terribly well: if there is an exception, the Lazy<> we did the
subscription with will still hold onto the exception, any use of the
Lazy then rethrows. This means StopFileChangeListening/Dispose never
actually clear the underlying cookie, and we crash in our Finalizer
assert later.

Raise events when connecting or disconnecting from a workspace for semantic changed

Merge features/dataflow to dev15.8-preview3

Fix for #2128 (Outlining cuts off closing } )