Fix webview focus issues that prevent cut/copy/paste when opening files with a quick pick

[tspence]

This PR fixes #116897.

When you use a full-screen webview, and then change focus away from it with a quick-pick (e.g. Cmd+P or Ctrl+P to open the file access quick pick), two different focus events happen rapidly:

• The quick pick tells the editor to open a new file, and
• The quick pick closes, which sends a focus message to the host telling it to return the focus back to the previously focused element.

Because webviews also have a special _focused flag, and because that flag is used to determine whether a webview should steal Cut/Copy/Paste commands, users can experience that cut/copy/paste no longer work in a second quickpick opened after the webview thinks it has focus.

This change prevents the webview from thinking it has focus when something else has taken place prior to the quick pick's close event.

[tspence]

This may also be related to #109874 and #114325.

I note that these two issues also include screenshots that demonstrate a webview in focus when the user reported the problem with a quick pick.

[mjbvz]

Thanks! Let's test this in insiders and see if it causes any issues. Currently on track for VS Code 1.54

[mjbvz]

I had to revert this due to a regression (#117595):

1. Open a md file
2. Open the preview to the side
3. Try using arrow keys to scroll the preview

The arrow keys no longer work since the webview is not focused.

@tspence Please look into revisiting when you have a chance

[tspence]

Thanks for the feedback! I'll see if I can investigate an alternative approach to solve this issue.

The root problem for me seemed to be:

1. Focus is on a webview
2. Open quick pick file selector
3. Choose one of the available files
4. Simultaneously: Quick pick closes and returns focus to webview; && quick pick selects a file and notifies VS Code to open and focus it.

In some cases when the webview is busy, it gets the notification to obtain focus after the quick pick has opened the new file. This seemed to affect us at FB more often because one of our webviews does lots of source control work.

A better fix might be to delay the "launch new file" action until after the quick pick has closed, or to make the quick pick not return focus to the previous webview if it took some action.

I'll consider what I can do and test something out using the #117595 issue you linked before resubmitting.