[Bug Report][3.7.3] v-combobox placeholder is not consistent with v-textbox

[giuliohome]

Environment

Vuetify Version: 3.7.3
Vue Version: 3.5.12
Browsers: Google Chrome, Microsoft Edge
OS: Windows, Linux

Steps to reproduce

A placeholder bound to an empty but non null model value

Expected Behavior

The combobx placeholder should show up as the v-text-field placeholder does

Actual Behavior

The placeholder remains hidden instead and strictly requires a null

Reproduction Link

https://play.vuetifyjs.com/#...

[MatthewAry]

May be related to #17122

[giuliohome]

Based on the playground, it appears that the regression was introduced in Vuetify 3.1.14 and was not present in 3.1.13 for the v-combobox.

And from 3.0.0-beta.9 to 3.0.0-beta.10 for the v-autocomplete.

[giuliohome]

Also the wrapInArray can be problematic, especially in conjunction with the logic around the model.value and how the empty values are handled. For null or undefined, wrapInArray returns an empty array []. When v is an empty string '', the wrapInArray function will wrap it in an array, i.e., wrapInArray('') will return ['']. So, if model.value is set to an array with an empty string [''], it won't trigger the same behavior as an empty model []. This can lead to the placeholder logic being broken because the model value isn't truly empty even if the input visually appears empty.

I see this already mentioned here but it was not related to v-combobox. However, the current problem with the v-combobox is that (here):

placeholder={ isDirty ? undefined : props.placeholder }

will incorrectly result in undefined for the placeholder when the model is initially empty. This contradicts the logic in onUpdateModelValue which correctly resets the model.value to [] also when v === ''.

Possible Fix (to be tested and verified):

const isDirty = model.value.length > 0 && !(model.value[0].value === '' && model.value.length === 1)