Slowly scrolling tabs bar with trackpad jittering

[brandonmcconnell]

Screen recording:

Screen.Recording.2023-07-18.at.7.06.40.AM.mov

This occurs when using either the external or built-in trackpad. There are no other input devices connected and in use when this occurs.

[bpasero]

This is not a regression and seems to reproduce for a very long time. If someone has a clue and wants to investigate, we apply the scroll state here to the tabs:

vscode/src/vs/workbench/browser/parts/editor/tabsTitleControl.ts

Lines 217 to 221 in 76415ef

	tabsScrollbar.onScroll(e => {
	if (e.scrollLeftChanged) {
	scrollable.scrollLeft = e.scrollLeft;
	}
	});

I feel the issue comes from our custom scrollable implementation, but cannot pin point it yet.

[bpasero]

I went deep into debugging this and feel this case cannot be fixed but maybe improved (in #189034). The tabs control scrollbar is configured in a way that both scrolling vertically and horizontally is allowed, scrolling horizontally. When using the trackpad I see myself often getting into a state where I end up scrolling both vertically and horizontally when scrolling slowly.

We do have logic to prefer the scroll direction with larger value, but in some cases the values are identical and as such compete. Today we pick one of the values but I think for the tabs stripe it would be better to ignore the scroll in such case.

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

Lines 393 to 399 in 268f349

	if (this._options.scrollPredominantAxis) {
	if (Math.abs(deltaY) >= Math.abs(deltaX)) {
	deltaX = 0;
	} else {
	deltaY = 0;
	}
	}

This would not fix stutter when slowly scrolling but I see it preventing weird direction turns of the stripe when scrolling slowly.

tl;dr; idea would be to ignore scroll events for scrollbars that map y to x where we see competing directions that would equal each other out.