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Fix opaque background on mermaid diagrams (#21642)

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Browsers introduce a opaque background on iframes if the iframe
element's color-scheme does not match the document's color scheme which
in case of a dark theme results in a mismatch and the browser adds a
white background. Avoid this by specifying the same color scheme outside
and inside the iframe.

See https://fvsch.com/transparent-iframes for more info.

My initial attempt was to make the iframe document the same color-scheme
as the parent page (light or dark) but with that, there was a ugly
background flash on load in Chrome because Chrome apparently always
loads iframe in light scheme initially. Firefox still shows a background
flash on load but this is not possible to get rid of and it's certainly
a browser bug.

Before:
<img width="1147" alt="Screen Shot 2022-10-31 at 13 30 55"
src="https://user-images.githubusercontent.com/115237/199017132-9828aace-bdd0-4ede-8118-359e72bcf2fe.png">

After:
<img width="1152" alt="Screen Shot 2022-10-31 at 13 30 36"
src="https://user-images.githubusercontent.com/115237/199017137-989a9e67-3fe0-445f-a191-df5bf290dabf.png">
This commit is contained in:
silverwind 2022-10-31 21:57:31 +01:00 committed by GitHub
parent 9a70a12a34
commit 57a0a7ef64
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2 changed files with 2 additions and 0 deletions
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1
web_src/js/markup/mermaid.js
View file

@ -2,6 +2,7 @@ import {isDarkTheme} from '../utils.js';
const {mermaidMaxSourceCharacters} = window.config; const {mermaidMaxSourceCharacters} = window.config;
const iframeCss = ` const iframeCss = `
:root {color-scheme: normal}
body {margin: 0; padding: 0} body {margin: 0; padding: 0}
#mermaid {display: block; margin: 0 auto} #mermaid {display: block; margin: 0 auto}
`; `;

1
web_src/less/markup/content.less
View file

@ -537,6 +537,7 @@
width: 100%; width: 100%;
height: var(--height-loading); // actual height is set in JS after loading height: var(--height-loading); // actual height is set in JS after loading
overflow: hidden; overflow: hidden;
color-scheme: normal; // match the value inside the iframe to allow it to become transparent
} }
.markup-block-error { .markup-block-error {