Fix block cursor obscuring placeholder text and editor text in some cases

[thataboy]

This PR tries to fix a couple of problems with the block cursor (and only the block cursor) not encountered before #17572 allowed user to change the cursor shape outside of vim mode and the terminal.

1. When the editor has placeholder text, the placeholder character is not shown because block_text does not look at placeholder text, only the display chars at the cursor.

2. block_text is displayed using EditorElement.style.background. This value is transparent black in some editors, causing regular text to be invisible. This happens with the filter text for the outline panel, for example

My fix for #2 is to use the theme's editor background instead. For the various built-in dark and light themes I tried, this seems to be a decent solution. I tried to use the inverse of the cursor color instead, namely h -> (h + 0.5) % 1., l -> 1. - l and other variations, but the result aren't great. Most themes favor a soft blue for the cursor and the inverse just ends up being soft brown and doesn't provide very good contrast. (Caveat: I have poor eyesight and this may be a me problem, but I think accessibility considerations apply.)

Here's how the fix using theme's editor background looks with gruvbox dark and Atelier Cave light

Release Notes:

• N/A

[iamnbutler]

This looks good to me, essentially we are just inverting the text color when using the block cursor in these cases?

I did just realize though–some themes set transparent background colors.

@thataboy Can we add a check to ensure that we only use editor_background if it isn't transparent? Otherwise we should use some value we know is not transparent (like pure black in a dark theme, pure white in a light theme.)

You can use something like cx.appearance() == Appearance::Dark or match on appearance to do that.

That is just a top of mind solution, there might be a better one though.

[iamnbutler]

Here editor_background may give you a value like hsla(0.,0.,0.,0.) in some themes. I assume this will make the text vanish, so we should have some sort of check to ensure this is a color that is at least partially opaque.

[iamnbutler]

We should have some sort of check to ensure the text color is at least partially opaque.

[thataboy]

This looks good to me, essentially we are just inverting the text color when using the block cursor in these cases?

I did just realize though–some themes set transparent background colors.

@thataboy Can we add a check to ensure that we only use editor_background if it isn't transparent? Otherwise we should use some value we know is not transparent (like pure black in a dark theme, pure white in a light theme.)

You can use something like cx.appearance() == Appearance::Dark or match on appearance to do that.

That is just a top of mind solution, there might be a better one though.

Ah thank you. I was wondering how to check if theme is light or dark.

Now set color according to Light or Dark appearance

[iamnbutler]

Let's try something like this?

// Invert the text color for the block cursor. Ensure that the text
// color is opaque enough to be visible against the background color.
//
// 0.75 is an arbitrary threshold to determine if the background color is
// opaque enough to use as a text color.
//
// TODO: In the future we should ensure themes have a `text_inverse` color.
let color = if cx.theme().colors().editor_background.a < 0.75 {
    match cx.theme().appearance {
        Appearance::Dark => Hsla::black(),
        Appearance::Light => Hsla::white(),
    }
} else {
    cx.theme().colors().editor_background
};

This fallback ensures a transparent background doesn't make it so the text disappears.

Without fallback:

With fallback:

[iamnbutler]

Thanks! Great change :)