Mark the current successor tab with [data-successor] attribute

[emvaized]

This pull request is a proposed solution for #1495

It adds [data-successor] attribute for a tab which is currently a successor for the current one (will be activated when the current one is closed). With the setting Tabs > After closing current tab: previously active tab it will add this attribute to the previously active tab, esentially replicating the "previous tab highlight" feature from the Arc browser.

Here's an example with Mouse > Tab actions > Activate previously active tab when clicking on the active tab (Tab flip) enabled:

2024-11-24.003712.mp4

Custom CSS rule used:

.Tab[data-successor="true"] .fav:before{
	content: "";
	position:absolute;
	background: var(--tabs-activated-bg);
	border-radius: var(--tabs-border-radius);
	box-shadow: var(--tabs-activated-shadow);
	outline: 1px solid lightgray;
	height: calc(16px + 10px);
	top: calc(-10px + 4px);
	width: calc(16px + 3px);
	left: -1.5px;
}

---

Another possible solution would be to use [data-last-active] attribute instead, and set it on every tab activation in onTabActivated. Probably it's a better solution to achieve the described behavior, but then it is not properly updated on tab close (newly focused tab remains with the last active attribute).

I got the impression that marking the successor tab works more reliably for this purpose in all usage scenarios I tested:

• on tab flip when click on the current tab
• on tab close when "activate previous tab" is enabled
• on current tab suspend