Thought I’d found a bug in WordPress, but it might be a Chrome vs. Firefox thing. I’m lazy, so I use a “Dashicon” for my custom block and the Placeholder component it uses. The Placeholder icon has a margin-right of 1ch, which at least in Chrome evaluates to 0, causing the Placeholder title to sit right next to the icon. (Current guess is the Dashicons font’s 0 has a width of zero, but that FF somehow falls back to a non-zero value. SVG icons work just fine.)
