src/vs/workbench/browser/parts/editor/sideBySideEditor.ts/SideBySideEditor/updateStyles
class SideBySideEditor:	override updateStyles(): void {
		super.updateStyles();

		if (this.primaryEditorContainer) {
			if (this.orientation === Orientation.HORIZONTAL) {
				this.primaryEditorContainer.style.borderLeftWidth = '1px';
				this.primaryEditorContainer.style.borderLeftStyle = 'solid';
				this.primaryEditorContainer.style.borderLeftColor = this.getColor(SIDE_BY_SIDE_EDITOR_VERTICAL_BORDER)?.toString() ?? '';

				this.primaryEditorContainer.style.borderTopWidth = '0';
			} else {
				this.primaryEditorContainer.style.borderTopWidth = '1px';
				this.primaryEditorContainer.style.borderTopStyle = 'solid';
				this.primaryEditorContainer.style.borderTopColor = this.getColor(SIDE_BY_SIDE_EDITOR_HORIZONTAL_BORDER)?.toString() ?? '';

				this.primaryEditorContainer.style.borderLeftWidth = '0';
			}
		}
	}