When dragging, the movement (transform: translate()) of split views that are not being dragged is not animated.

I don't understand what you're expecting to happen with regards to animation. Tabs in a splitview need to stay in their container - they should always move together - and that's been accounted for in the drag and drop code, per the bug 1986948.

I mean, during dragging, the background tabs shift with a smooth transition, but the split views don't. They just jump to the position instantly.

There's a transition: var(--tab-dragover-transition) set for the non-dragging tabs:

Nightly 148.0a1 (2026-01-06)

/* tabs.css lines 259 */
.tabbrowser-tab {
  /*......*/
  #tabbrowser-tabs[movingtab] & {
    position: relative;

    @media (prefers-reduced-motion: no-preference) {
      :not(tab-group:active) > &[fadein]:not(:active, [multiselected]),
      &[multiselected-move-together],
      &[tabdrop-samewindow] {
        transition: var(--tab-dragover-transition);
      }
    }
  }
  /*......*/
}

And even the tab group labels:

/* tabs.css lines 1563 */
.tab-group-label-container {
  /*......*/
  @media (prefers-reduced-motion: no-preference) {
    #tabbrowser-tabs[movingtab] &:not(:active) {
      transition: var(--tab-dragover-transition);
    }
  }
}

And I'm just wondering why on earth split views are considered so special that they're expected never to be applied with such --tab-dragover-transition?

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:sclements, could you have a look please?

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(In reply to Merci chao from comment #2)

I mean, during dragging, the background tabs shift with a smooth transition, but the split views don't. They just jump to the position instantly.

There's a transition: var(--tab-dragover-transition) set for the non-dragging tabs:

Nightly 148.0a1 (2026-01-06)

/* tabs.css lines 259 */
.tabbrowser-tab {
  /*......*/
  #tabbrowser-tabs[movingtab] & {
    position: relative;

    @media (prefers-reduced-motion: no-preference) {
      :not(tab-group:active) > &[fadein]:not(:active, [multiselected]),
      &[multiselected-move-together],
      &[tabdrop-samewindow] {
        transition: var(--tab-dragover-transition);
      }
    }
  }
  /*......*/
}

And even the tab group labels:

/* tabs.css lines 1563 */
.tab-group-label-container {
  /*......*/
  @media (prefers-reduced-motion: no-preference) {
    #tabbrowser-tabs[movingtab] &:not(:active) {
      transition: var(--tab-dragover-transition);
    }
  }
}

And I'm just wondering why on earth split views are considered so special that they're expected never to be applied with such --tab-dragover-transition?

Thanks for clarifying. Noone is saying it shouldn't have that style applied but I couldn't see from the video and description what the issue and expectation was. I think the rule probably needs to be adjusted for tabs in the split-view wrapper.

I might see if I can incorporate this into follow up bug 2003764, otherwise I'll give it a 'good-first-bug' or 'helpwanted' keyword as we can always use help with smaller bugs like this.