src/vs/workbench/contrib/terminal/browser/terminalInstance.ts/TerminalInstance/_getDimension
class TerminalInstance:	private _getDimension(width: number, height: number): ICanvasDimensions | undefined {
		// The font needs to have been initialized
		const font = this._configHelper.getFont(this._xtermCore);
		if (!font || !font.charWidth || !font.charHeight) {
			return undefined;
		}

		if (!this._wrapperElement) {
			return undefined;
		}

		const wrapperElementStyle = getComputedStyle(this._wrapperElement);
		const marginLeft = parseInt(wrapperElementStyle.marginLeft!.split('px')[0], 10);
		const marginRight = parseInt(wrapperElementStyle.marginRight!.split('px')[0], 10);
		const bottom = parseInt(wrapperElementStyle.bottom!.split('px')[0], 10);

		const innerWidth = width - marginLeft - marginRight;
		const innerHeight = height - bottom - 1;

		TerminalInstance._lastKnownCanvasDimensions = new dom.Dimension(innerWidth, innerHeight);
		return TerminalInstance._lastKnownCanvasDimensions;
	}