Unwanted selections don't switch to Visual Mode

I'm not sure if this behavior is ever desirable, but created this
PR anyway so that we can discuss.

I don't see any downsides from doing this. Visual mode works from
key input and mouse selections. We just don't automatically switch
to Visual Mode when Visual Studio decides that it wants to select some
text for us by itself.

Fixes #2966

Src/VimCore/SelectionChangeTracker.fs

@@ -147,24 +147,22 @@ type internal SelectionChangeTracker
                     else
                         Some ModeKind.SelectCharacter
                 elif _textView.Selection.Mode = TextSelectionMode.Stream then 
-                    let modeKind = 
-                        match _vimBuffer.ModeKind with
-                        | ModeKind.VisualCharacter -> ModeKind.VisualCharacter
-                        | ModeKind.VisualLine -> ModeKind.VisualLine
-                        | ModeKind.VisualBlock -> ModeKind.VisualCharacter
-                        | ModeKind.SelectCharacter -> ModeKind.SelectCharacter
-                        | ModeKind.SelectLine -> ModeKind.SelectLine
-                        | ModeKind.SelectBlock -> ModeKind.SelectCharacter
-                        | _ ->
-                            // We were not already in a visual mode and the
-                            // user did not initiate the selection with the
-                            // mouse.  In that case handle the external select
-                            // by using the 'selectmode=mouse' setting
-                            if isSelectModeMouse then
-                                ModeKind.SelectCharacter
-                            else
-                                ModeKind.VisualCharacter
-                    Some modeKind
+                    match _vimBuffer.ModeKind with
+                    | ModeKind.VisualCharacter -> Some ModeKind.VisualCharacter
+                    | ModeKind.VisualLine -> Some ModeKind.VisualLine
+                    | ModeKind.VisualBlock -> Some ModeKind.VisualCharacter
+                    | ModeKind.SelectCharacter -> Some ModeKind.SelectCharacter
+                    | ModeKind.SelectLine -> Some ModeKind.SelectLine
+                    | ModeKind.SelectBlock -> Some ModeKind.SelectCharacter
+                    | _ ->
+                        // We were not already in a visual mode and the
+                        // user did not initiate the selection with the
+                        // mouse.  In that case handle the external select
+                        // by using the 'selectmode=mouse' setting
+                        if isSelectModeMouse then
+                            Some ModeKind.SelectCharacter
+                        else
+                            None
                 else 
                     // Handle TextSelectionMode.Box cases
                     if _vimBuffer.ModeKind = ModeKind.SelectBlock then
@@ -175,9 +173,10 @@ type internal SelectionChangeTracker
                         Some ModeKind.SelectBlock
                     else
                         Some ModeKind.VisualBlock
+
             match desiredNewMode with 
-            | None -> None
-            | Some kind -> if kind <> _vimBuffer.ModeKind then Some kind else None 
+            | Some kind when kind <> _vimBuffer.ModeKind -> Some kind 
+            | _ -> None 
 
         // Update the selections.  This is called from a post callback to ensure we don't 
         // interfer with other selection + edit events.