3.4.15 regression on v-model in <select>

[jacekkow]

Vue version

3.4.15

Link to minimal reproduction

https://gist.github.com/jacekkow/9a494e92ca45d16907d2a2514e8422c2

Steps to reproduce

Click on "First (click)" and/or "Second (click)".

What is expected?

<option> tags within <select> have the "selected" flag set. This happens when downgrading to Vue 3.4.14.

What is actually happening?

<option> within <select> does not have the "selected" flag set.

System Info

No response

Any additional comments?

No response

[LinusBorg]

Cuplrit:

core/packages/runtime-dom/src/directives/vModel.ts

Lines 242 to 245 in f1068fc

	// fast path for updates triggered by other changes
	if (isArrayValue && looseEqual(value, oldValue)) {
	return
	}

value and oldValue are the same array. So that fastpath doesn't work on mutations to the select array. It doesn't cause a problem when selecting an item in the select itself because then we skip the that part of the code.

Introduced by 2ffb956

a workaround until a fix would be to use an immutable array.

But the question is if/how we can keep the fastpath goal of that commit while supporting mutating the array, which is kind of a common thing to do and should of course work.

[Doctor-wu]

reproduction in playground