[MarkFonts]

Fixes #159.

In #159, font-optical-sizing: auto never took effect. That came from two separate problems, and this PR fixes both.

1. Slider values stuck after switching to "[Inherit page styles]"

service-worker.js kept track of the CSS it had injected into each tab in a module variable (injectedCSS). Chrome stops idle MV3 service workers after about 30 seconds, and that variable is lost when it does. The next update then inserts the new stylesheet but never removes the old one. So a stylesheet with the slider values ('opsz' 18, 'wght' 300, …) stays in the page after switching to Inherit, and the explicit 'opsz' overrides automatic optical sizing. Restarting Chrome clears it, which made the bug look intermittent.

• Track the injected CSS in chrome.storage.session, which survives worker restarts.
• Put CSS updates in a queue so quick successive changes can't orphan a stylesheet.
• Remove a tab's entry when the tab closes.
• If writing to session storage fails (10 MB quota, and font files are inlined), log it at debug level. The worst case is the current behavior.

2. No way to leave optical size on auto while using the sliders

Once any slider was touched, cssVariationSettings() wrote every axis, including opsz. The only way back to automatic optical size was "[Inherit page styles]", which also drops your other axis choices. The workaround was a hand-written font-variation-settings in the CSS box that left opsz out.

• The Optical size value now doubles as a toggle, styled like the upload button. It reads AUTO in the accent color, or shows the number when it's off. While it's on, opsz is left out of font-variation-settings, so the browser matches it to the font size.
• Auto is on by default for new fonts. Moving the slider turns it off, and clicking the toggle turns it back on. It's keyboard accessible (role="button", Enter/Space, aria-pressed).
• New field Font.autoOpsz. Fonts saved before this change read it as true. Decision for you: that means anyone who had deliberately set opsz with a slider gets auto after updating. If you'd rather keep their setting, change the default in Font.fromObject to false.

Testing

I tested headlessly in Chromium with the built extension, using Cal Sans VF (opsz 8–45) on a <p> at 44px.

Scenario	computed font-variation-settings
main: sliders at opsz 8 → worker stopped → Inherit	"opsz" 8, "wght" 400 (stale)
this PR: same steps	normal
without the worker restart, both builds	normal
sliders, Auto on	"GEOM" 25, "wght" 400 (no opsz)
sliders, Auto off	"GEOM" 25, "opsz" 8, "wght" 400
font saved before this change (no autoOpsz)	"GEOM" 25, "wght" 400

Popup: the toggle reads "auto" on open. Moving the slider to 30 shows "30" and stores autoOpsz: false. Clicking the toggle brings back "auto" and true, and the state survives reopening the popup. Only the opsz row gets the toggle.

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