Bring back ballistic scroll for mouse events

[Tyriar]

In 721d483 after the new viewport was added I made mouse wheel events a single up or down sequence as the lines scroll logic was no longer in the codebase. We should bring it back such that ballistic scroll works in the alt buffer.

Old lines scrolled code:

xterm.js/src/browser/Viewport.ts

Lines 349 to 366 in 9ba6c00

	public getLinesScrolled(ev: WheelEvent): number {
	// Do nothing if it's not a vertical scroll event
	if (ev.deltaY === 0 || ev.shiftKey) {
	return 0;
	}
	// Fallback to WheelEvent.DOM_DELTA_LINE
	let amount = this._applyScrollModifier(ev.deltaY, ev);
	if (ev.deltaMode === WheelEvent.DOM_DELTA_PIXEL) {
	amount /= this._currentRowHeight + 0.0; // Prevent integer division
	this._wheelPartialScroll += amount;
	amount = Math.floor(Math.abs(this._wheelPartialScroll)) * (this._wheelPartialScroll > 0 ? 1 : -1);
	this._wheelPartialScroll %= 1;
	} else if (ev.deltaMode === WheelEvent.DOM_DELTA_PAGE) {
	amount *= this._bufferService.rows;
	}
	return amount;
	}

[eddyb]

Thanks! If this does get resolved, care should be taken to re-open (or at least check if the problem comes back):

• Terminal scrolling regression in application with alternative screen buffer microsoft/vscode#204769

(My best understanding of what happened in 1.92.0 is that on certain setups this change can be e.g. 7x slower to scroll, which accidentally hides other potential effects of real-time scrolling)