chore: remove underscores and clean up scrollbar.ts

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

Part of #6548

Proposed Changes

In three commits:

• Restructure the constructor so that properties are set in the constructor, not in a helper function. This let me remove nullability from several SVGElements that the scrollbar tracks.
• Stop tracking an SVG Element that was only used during construction.
• Change dispose to remove the scrollbar's root node but not set anything else to null. This also helped with nullability. The garbage collector is in charge of the rest of cleanup.
• Add comments to property definitions to make them more meaningful for the next reader.
• Rename all private properties and methods that end in underscores to remove the underscores.

Behavior Before Change

No change.

Behavior After Change

No change.

Reason for Changes

Cleanup, and reading code to understand dependencies.

Test Coverage

Mocha tests, but I also opened up the playground and both dragged and clicked scrollbars in several situations:

• LTR, desktop
• LTR, touch
• RTL, desktop
• RTL, touch

Documentation

None

Additional Information

[cpcallen]

The comment is not very interesting, and probably needn't have been preserved.

[rachel-fenichel]

Removed.

[cpcallen]

Opinion: I would probably not have bothered extracting this as a separate method.

[rachel-fenichel]

Extracting it makes the constructor fit in a single window for me without rotating my monitor :)

I actually would rather have extracted the lengthAttribute/positionAttribute setting as well, but doing it directly in the constructor means that the compiler is certain about nullability.

[cpcallen]

Aside: how did this not fail our format check??

[cpcallen]

Oh, huh. It looks fine here, but in the side-by-side diff it looked like it was previously:

   if (this.svgHandle_) {
    this.workspace.getThemeManager().unsubscribe(this.svgHandle_);
  }
  }