vscode.workspace.textDocuments call does not return hidden previewHtml documents

[Raymondd]

• VSCode Version: 1.9.0
• OS Version: Win10 and MacOS

Steps to Reproduce:
Configuration to connect to SQL from the extension
Reference: #19732

Repro steps - execute 2 separate queries
Highlight the text select * from sys.databases and execute MS SQL: Execute Query (Ctrl+Shift+E) to run
You will notice the results editor tab opens, which is a WebView component.

Hide this webview behind the .sql document you just created:

Highlight the text select * from sys.tables and execute the MS SQL: Execute Query

Expected:
The results editor tab is found, brought to the foreground, and refreshed.

Actual:
The results editor tab is not found and not refreshed. A new results tab with the same URI is opened in the second pane.

Note that, if you have the results tab in the foreground, it is always found and refreshed properly.
The reason this is not happening is because this slice of code tells us if there is one already open or not:

    public doesResultPaneExist(resultsUri: string): boolean {
        let resultPaneURIMatch = vscode.workspace.textDocuments.find(tDoc => tDoc.uri.toString() === resultsUri);
        return (resultPaneURIMatch !== undefined);
    }

vscode.workspace.textDocuments contains all text documents, regardless of if they are visible or not, but it does not contain all previewHtml documents. It only contains the previewHtml documents that are visible. (In other words, any previewHtml documents which are not visible are not found in vscode.workspace.textDocuments which is inconsistent with other types of text documents)

[mjbvz]

@bpasero can you please take a look.

This issue also effects the markdown preview (see issues like #8581). When I looked through the implementation in VSCode, I believe the current behavior may have been by design but perhaps we need a way to return all documents including html previews.

[jrieken]

Closing this as dupe of #15178 which is about knowing what tabs (tab != editors != documents) are open. Ultimately we should migrate folks from the dark side and enable actual UX extensions