{#if}/{:else} blocks should preserve order during transitions

[adiguba]

Describe the problem

When the condition of an {#if}/{:else} is updated, the content of the blocks are removed/added into the DOM.
But the order of the blocks are not preserved during transitions : the new visible block is always added after the previous visible block.

I think it would make more sense to preserve the order of the if/else.

Exemple : Push the button toggle and wait for the end of the transition. Push it again and the if block will pop from the bottom.
https://svelte.dev/playground/hello-world?version=5.15.0#H4sIAAAAAAAACp1Sy27bMBD8FZYOEAtxYqePQxVJQNFL-w11DxS5FAhTpECu3LiC_r0kJdl5XgodJC1nZ3aHM1DDWqA5_QFaW_LHOi3IGoRCEBndUKk0eJr_GiieuoiLhVCfu7513Z0_gsZYq5mHt-rcGgSDgYYWnjvVYbU3e1RtZx2SgXitBJCRSGdbcj21bdEx4xUqa64jWAOSo_Kq1kBKcuWRIazR9ZA9hOMACCIeie1ihw-QQfSOxZ-c3O92uzHAiu1F3RR1j2gNsYZrxQ_lsM7KalYoP8wfY4W2aTQU2wmdOoeVksssY5QuhDqSy7x52qcc5llG4vGkIa8ZPzTO9kaUe7qqmZT8654mJ4IXMhFtA1OoDDlo___cUtafaha4I8uZc6vkmKxKT5Eak_psxDBNIpTvNDvlpNaWHx6mYseEUKbJye7uC7TkHtr5oGWuUWapsx5tOkijx8ln0to6AS7cRPdIvA0rkBXn_CX5R3fmlSEyt179hWdVhEe8ZVo1QZGHRIFb1MLVLvuYkLgIpHmMx7h5J7nfg1uB4M30vjp7J8ExlDxiL5HcLXm8AimB43qdkbJafJhCeghwD_gzahyZniGJ6OZmk-KaTXgH2DtDZoAG5s5Nh2zafXq9Cnc1JL4xJLd6ed_LNE9M_jx7_MTK5zb-Hv8Bcz2ulioEAAA=

Describe the proposed solution

Technically the solution seems simple, since it would be enough to change this line :

svelte/packages/svelte/src/internal/client/dom/blocks/if.js

Line 72 in 1d773ef

consequent_effect = branch(() => fn(anchor));

	consequent_effect = branch(() => fn(alternate_effect?.nodes_start ?? anchor));

But it changes the behavior and I don't know how to estimate the impact of that.

Importance

nice to have

[Conduitry]

I don't think there's one behavior that's more obviously correct than the other one. Changing this would be a breaking change for anyone who happens to be relying on the current behavior. If you need more explicit control over this, you can use separate {#if}/{/if} blocks.