Implement tab scrolling on tabbar

[krassowski]

As a solution to https://github.com/jupyterlab/jupyterlab/issues/10305 I would like to propose adding (optional) tabbar scrolling.

Downstream applications need to define min-width on tabs for scrolling to work reasonably.

Associated JupyterLab PR: https://github.com/jupyterlab/jupyterlab/pull/12278

TODO:

• [x] scroll activated tab into view when switching tabs or opening a new tag
• [x] fix tests
• [ ] replace div buttons with proper buttons?
• [ ] hoist scrolling rate and interval to options, or CSS?
• [ ] add tests
• [ ] scrolling to active tab should not happen when closing another tab

User facing changes

Users can scroll:

• by pressing the arrow buttons at the ends of tabs list (which only show up if the tab bar overflows)
• by using browser built-in scrolling (e.g. Shift + mouse wheel, or just a wheel)
• when dragging tabs by hovering over the arrow buttons

Backward compatibility

One wrapper div was added to the DOM hierarchy which may invalidate some some of the downstream CSS styles (if they use strict > operator). Otherwise this PR aims to be backward compatible.

[ajbozarth]

Adding my overall review here (I tested both PRs together):

Testing this locally I've found a few issues/oddities:

• The scroll on my Mac trackpad is reversed when scrolling (this may be that lab is ignoring a system setting for scroll direction on a trackpad). My scroll is set to behave like a phone but it's behaving like a mouse wheel. Also the scroll is inconsistent in behavior, it sometime starts to scroll then stops (again a trackpad thing, usually happens when I do short quick scrolls, which I do often).
• The scrolling is "fluid" unlike how Firefox does it. In FF clicking the button hard scrolls the approx. length of a tab, whereas this scrolls like a webpage scrollbar. I believe the "hard pop" from tab to tab like in FF is a better UX.
• There's now a gap between the tab and the main area that wasn't there before, you can see the difference in screenshots posted by @krassowski and @aiqc in https://github.com/jupyterlab/jupyterlab/pull/12278
• The "fade" around the scroll button is a bit tall for some reason
• On the note of default tab width I think we could get away with it being the width of the icon set (ie the tab width if you ignore the label). That way the icon, close button, and kernel icon will all be visible
• On a positive note I like the size of the scroll buttons, they look good

Note that I tested this on Safari 15.3

I'll come back to test this a bit more later, as for the code I saw no large issues with it, but will admit the scroll handling code was a bit dense to understand since I haven't dealt with scroll handling in years

[ajbozarth]

A code point I forgot: Is this still considered backwards compatible if we change the DOM structure? I'm not sure how detailed jupyterlab defines API breaks, does it include changes to the DOM? If so would this even be valid for lab 3.4?

[krassowski]

There's now a gap between the tab and the main area that wasn't there before, you can see the difference in screenshots posted by krassowski and aiqc in https://github.com/jupyterlab/jupyterlab/pull/12278

@ajbozarth I don't see this. Could you please circle the fragment you are referring or link before-after pictures?

[ajbozarth]

Before: Note how the tab and the empty toolbar are all one continuous "block" compared to where there is a line/border between the tab and toolbar section. That "line" should be there for tabs in the background, but not the active tab

[krassowski]

The scroll on my Mac trackpad is reversed when scrolling (this may be that lab is ignoring a system setting for scroll direction on a trackpad). My scroll is set to behave like a phone but it's behaving like a mouse wheel. Also the scroll is inconsistent in behavior, it sometime starts to scroll then stops (again a trackpad thing, usually happens when I do short quick scrolls, which I do often).

This could be solved by using a dummy element which intercepts scroll events as in https://github.com/jupyterlab/lumino/pull/652 - which is the recommended approach (as compared to using mouse wheel event as this PR attempted).