Transition bug (in, out + derived state) (derived intro/outro & params): opacity leak between intro and outro. Reactivity or outro animation issue.

[andriytyurnikov]

Describe the bug

I am using in: and out: with derived state for maximal reactivity.
Unfortunately there is a weird bug, which either rooted in issues with reactivity, or outro animation mechanics, as opacity animation happens where it should not.

Reproduction

Link to REPL:
https://svelte.dev/playground/4fb3afd837fe4100b40d1a7924dcdd15?version=5.25.6

Logs

System Info

System:
    OS: macOS 15.3.2
    CPU: (8) arm64 Apple M2
    Memory: 85.06 MB / 24.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - ~/.local/share/mise/installs/node/22.14.0/bin/node
    npm: 10.9.2 - ~/.local/share/mise/installs/node/22.14.0/bin/npm
    pnpm: 10.5.1 - ~/Library/pnpm/pnpm
    bun: 1.2.6 - ~/.local/share/mise/installs/bun/1.2.6/bin/bun
  Browsers:
    Chrome: 134.0.6998.166
    Safari: 18.3.1
  npmPackages:
    svelte: ^5.25.6 => 5.25.6

Severity

annoyance

[andriytyurnikov]

Similar to #15701

z-index management issue