[Bug Report][3.9.6] v-combobox placeholder not showing initially if the model value is empty string

[Cleric-K]

Environment

Vuetify Version: 3.9.6
Vue Version: 3.5.20
OS: Windows 10 (current)

Steps to reproduce

1. See the link. The placeholder should be visible on page load but it is not.

Expected Behavior

The placeholder should be visible after the page is initially opened (on focus or always, if persistent-placeholder is set)

Actual Behavior

The placeholder is missing when the page opens. If I enter a value and then clear it, the placeholder now shows as it should.

If instead of empty string, the initial v-model value (the ref) is set to null, the placeholder shows properly.

Reproduction Link

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

Images

[jcjp]

Looks like this is an intended behavior the empty string '' is considered not an empty state, only null and undefined would be. Is there a use case where you would need to use '' to show the placeholder why not use null or undefined?

[Cleric-K]

There's a misunderstanding here. I follow the PR and what @J-Sek says there concerns a <v-combobox> in multiple mode. I never suggested that [''] should be considered empty. Besides, when the value is [] the placeholder works as it should - it shows on the initial render, so no problems there (example).

The bug concerns the normal (non-multiple) mode. It makes no sense not to show the placeholder when the initial model value is ''. Furthermore, it is obvious that the behavior is inconsistent because, when I open the initial form and the placeholder is not showing (value ''), then I type a character (value 'a'), then I hit backspace (value again ''), the placeholder now shows.

And even further, such 'intended' behavior is inconsistent with the behaviors of other widgets, like <v-text-field>. When their model value is '' on initial render they show the placeholder just fine (example - here both inputs use the same model).

BTW, I noticed an interesting behavior with this last example. If edit the value in the <v-text-field> and then erase it again, the placeholder in the <v-combobox> still doesn't show. It is only if I edit and erase the value within the latter that its behavior switches and begins to show properly the placeholder when the value is ''.

[jcjp]

Okay I understand, the code by default it seems to put the values in [] let me double check this. Here is the value if you pass an '':

As you can see it is an array of objects, CC: @J-Sek how should we move forward with?

[Cleric-K]

So if I understand correctly, the problem comes from the fact that a combobox is internally always multiple, and the ordinary mode is just a special case where it is a multiple with only one value.

I wonder what happens when a character is typed and then erased. Would the internal representation become []? And how come this is still converted to model value '' in the outside world? Maybe we can implement something similar in reverse? On init (if the input is not multiple), instead of simply taking the initial model value and wrapping it in the internal array, leave that array empty if the model is empty string. This seems reasonable because otherwise we have different behaviors depending on the direction.
Model '' -> Internal ['']
Internal [] -> Model ''
i.e. we have two different internal states that both equate '' in the model.