Scrollbar size settings are "dendrified"

[per1234]

Bug Description:

When a non-default value is set for the editor.scrollbar.horizontalScrollbarSize or editor.scrollbar.verticalScrollbarSize settings, the default value is still used for all new editors/windows.

Steps to Reproduce:

1. Run the browser example.
2. Select File > New File from the menus.
3. Hold the Space key until a horizontal scrollbar appears.
4. Hold the Enter key until a vertical scrollbar appears.
5. Open the Command Palette.
6. Execute the "Preferences: Open Settings (JSON)" command.
7. Set the editor.scrollbar.horizontalScrollbarSize and editor.scrollbar.verticalScrollbarSize settings to 50:

   {
       "editor.scrollbar.horizontalScrollbarSize": 50,
       "editor.scrollbar.verticalScrollbarSize": 50,
   }

8. Select the previously created editor.
   🙂 The horizontal and vertical scrollbar "troughs" are sized according to the custom setting.
   ❗ There is an unrelated bug that causes the "thumbs" to remain at the default size.
9. Select File > New File from the menus.
10. Hold the Space key until a horizontal scrollbar appears.
11. Hold the Enter key until a vertical scrollbar appears.

🐛 The horizontal and vertical scrollbar "troughs" are sized according to the default setting (14).

Additional Information

• Operating System: Windows 10
• Theia Version: d3f6359

Related: #11709

[colin-grant-work]

The reason those settings are not applied is that those values are currently hard-coded in monaco-editor:

theia/packages/monaco/src/browser/monaco-editor.ts

Lines 136 to 143 in 205193d

	scrollbar: {
	...options?.scrollbar,
	useShadows: false,
	verticalHasArrows: false,
	horizontalHasArrows: false,
	verticalScrollbarSize: 10,
	horizontalScrollbarSize: 10
	}

In my earlier PR, I chose not to modify that in case there was a good reason for having those values set that way, but either removing the hard-coded values or switching the spread to the end of the object would allow those preference values to take effect in new editors. Moving the spread to the bottom is probably the best approach.

[ravitejasssihl]

Hey @colin-grant-work can you give me a bit more idea on what changes are you expecting? I am a complete beginner and would like to up the issue.

[colin-grant-work]

@ravitejasssihl, basically, at the moment, we're spreading the user-supplied options, then providing defaults. That means that anywhere the defaults conflict with user options, the defaults win, which isn't desirable. Switching the code here to something like this would allow the user options to win out:

        const combinedOptions = {
            lightbulb: { enabled: true },
            fixedOverflowWidgets: true,
            ...options,
            scrollbar: {
                useShadows: false,
                verticalHasArrows: false,
                horizontalHasArrows: false,
                verticalScrollbarSize: 10,
                horizontalScrollbarSize: 10,
                ...options?.scrollbar,
            }
        } as IStandaloneEditorConstructionOptions;

[per1234]

@svenefftinge I see you were the one who set the scrollbar widths to this nonstandard size they are now locked to: #852

Would you be able to clear up @colin-grant-work's doubt about whether the locking of the setting was done intentionally:

I chose not to modify that in case there was a good reason for having those values set that way