Fix strange behavior when scrolling downward at low-speed with trackpad

[Chigusa0w0]

This PR fixes microsoft/monaco-editor#2623 and similar misbehavior in VSCode up to 1.60.0-insider (i.e., editor will not respond to low-speed two-finger scrolling).

When scrolling with trackpad two-figure scrolling, or in the ending phase of an inertial scrolling, small floating pointing numbers of deltaX/deltaY will be fed to the handler. When the number is small enough, the scrollTop/scrollLeft will change by a value less than 1.

vscode/src/vs/base/browser/ui/scrollbar/scrollableElement.ts

Lines 411 to 421 in 44129da

	if (deltaY) {
	const desiredScrollTop = futureScrollPosition.scrollTop - SCROLL_WHEEL_SENSITIVITY * deltaY;
	this._verticalScrollbar.writeScrollPosition(desiredScrollPosition, desiredScrollTop);
	}
	if (deltaX) {
	const desiredScrollLeft = futureScrollPosition.scrollLeft - SCROLL_WHEEL_SENSITIVITY * deltaX;
	this._horizontalScrollbar.writeScrollPosition(desiredScrollPosition, desiredScrollLeft);
	}
	// Check that we are scrolling towards a location which is valid
	desiredScrollPosition = this._scrollable.validateScrollPosition(desiredScrollPosition);

Unfortunately, the validateScrollPosition method, which internally calls new ScrollState(), will simply round-down the input value.

vscode/src/vs/base/common/scrollable.ts

Lines 64 to 69 in 8be960c

	width = width | 0;
	scrollWidth = scrollWidth | 0;
	scrollLeft = scrollLeft | 0;
	height = height | 0;
	scrollHeight = scrollHeight | 0;
	scrollTop = scrollTop | 0;

Therefore, for upward scrolling, users can get a smooth scrolling experience. However, for downward scrolling, VSCode/Monaco will not respond to such scrolling requests when its speed is below a certain level. With the default settings, the speed threshold is relatively high. In some cases, the difficulty of scrolling down in VSCode/Monaco is quite noticeable. For Monaco, this also means that under certain configurations such low-speed scroll events are passed to other HTML elements, causing unexpected scrolling of other elements.

This PR ensures that low-speed scrolling events from user/inertial scrolling are correctly responded to by introducing active symmetric rounding (round away from zero). The behavior is consistent with the original VSCode/Monaco upward scrolling behavior. No disruptive changes to the user experience are expected.

[alexdima]

Thank you!