Forms: use a placeholder attribute in the editor instead of value

[simison]

Use a placeholder attribute in the editor for placeholder text instead of value attribute.

By using value, we have a couple issues.

When theme includes styling for ::placeholder pseudo element, we don't show it. Consider for example CSS:

input::placeholder {
	font-style: italic;
}

Before

After

When we add a number input, its type="number" disallows non-numeric value, even if placeholder could be numeric. We would only need to support numeric placeholders for that input. We could change that field to type="text" in the editor, but that would miss out browser number controls, any styling applied to number fields, and otherwise also mismatch what they see in the editor with frontend.

Before

After

Proposed changes:

• Use a placeholder attribute in the editor for placeholder text instead of value attribute

Other information:

• Have you written new tests for your changes, if applicable?
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Testing instructions:

• Add different inputs and set their placeholders; they continue working.

[github-actions]

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Code Coverage Summary

This PR did not change code coverage!

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Full summary · PHP report · JS report

[enejb]

@simison Does this work as expected for you in any text input field?
I am not able to get it to work. It replaces the first character only.

[lezama]

makes sense

[lezama]

works ok for me

It replaces the first character only.

what do you mean? 😆

[edanzer]

I see what @enejb is talking about. With this PR loaded, I can add placeholder text as expected in the sidebar. But if I try to type placeholder text directly into the input in the main part of the block editor, it always deletes all but the last character I typed. I also confirmed this does not happen on trunk. I'm sure it's just some mix-up of state - basically since we changed the relationship between value and placeholder, we don't track value properly when typing into the input.

[simison]

Weird, that's the scenario that worked for me just fine, but I'll do further testing 👍 Might be browser dependant.

[edanzer]

Just to be sure we're talking and seeing the same thing, here's a video of the behavior. I type first in the placholder field in the sidebar (works), and then directly in the input.

I've also confirmed that that if I keep value = { placeholder } like below, the issue is resolved

value={ placeholder }
placeholder={ placeholder }

input-placeholder.mp4

[simison]

I've also confirmed that that if I keep value = { placeholder } like below, the issue is resolved

Thanks! It might make sense to retain value when focused on the input, and set it empty when not. 🤔 That would fix the styling (keeping value removes the styles), and would still likely need some changes for inputs like Number where value can be only number.