twbs / twbs/bootstrap

Can not trasition opacity for .collapse element

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Description

Prerequisites
Describe the issue

I am aware of the problem that opacity is not animated when the element has display: none.
So I tried to work around the problem with this code:

      $( '#xxx' )
        .on('show.bs.collapse',  function ( e ) {
          e.target.style.opacity= 0;
          e.target.offsetHeight;      // Force reflow
          e.target.style.opacity= 1;
        })
        .on('shown.bs.collapse', function ( e ) { e.target.style.opacity= ''; })

When an element collaped it is defined at BS as the next:

.collapse:not(.show) {
  display: none;
}

Thus when the element has .collapse it is not displayed yet.

So when I checked BS source code:
image

the show.bs.collapse event is called (1) before it becomes displayed (2).

To my mind the show.bs.collapse should be called when the element is actually visible. But having this I see other issue:
a) element becomes visible with default properties
b) event is called
c) event handler sets required properties
d) element blinks for a short period of time
So this is not the solution.

The possible solution could be to fire events in the next order before.show.bs.collapse then show.bs.collapse and shown.bs.collapse events.

In this case I will be able to force reflow.

Reduced test cases

I do not have one.

What operating system(s) are you seeing the problem on?

Linux

What browser(s) are you seeing the problem on?

Firefox

What version of Bootstrap are you using?

v5.3

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

The issue points to Bootstrap's Collapse source around the show.bs.collapse event; start by reproducing the behavior in Firefox with a minimal test case and tracing that event's ordering. Done means the event lifecycle and opacity-transition behavior are specified and covered by an appropriate regression test.

Written by the indexing model from the issue text.

Assessment

Tech stack
bootstrap, css, javascript
Domain
frontend
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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