That nearly-transparent color was indeed added in the regressing commit, here: https://hg.mozilla.org/integration/autoland/rev/89da90df398f5aee595e14748b0241ede2a9fd4d#l1.40 The issue here seems to be a set of assumptions-mismatches: (A) It looks like this commit 89da90df398f5aee595e14748b0241ede2a9fd4d is assuming that this nearly-transparent color is OK as a background-color because it'll only be used for backgrounds (and presumably we're assuming that, as a background, it'll be layered on top of some other background-color that provides sufficient contrast). (B) The translations code is assuming that it can swap the background/foreground colors here and still have reasonable contrast (note in the first code-snippet in comment 4, we've got a `fill` color-variable as our background, and a `background` color-variable as our foreground). These assumptions are in conflict, which is what's causing this issue.
Bug 1879415 Comment 5 Edit History
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That nearly-transparent color was indeed added in the regressing commit, here: https://hg.mozilla.org/integration/autoland/rev/89da90df398f5aee595e14748b0241ede2a9fd4d#l1.40 https://searchfox.org/mozilla-central/rev/54c9b4896fdc1e858cd4942f306d877f1f3d195e/browser/themes/linux/browser.css#35 ``` --toolbar-field-background-color: light-dark(rgba(0, 0, 0, .05), rgba(0, 0, 0, .3)); ``` The issue here seems to be a set of assumptions-mismatches: (A) It looks like this commit 89da90df398f5aee595e14748b0241ede2a9fd4d is assuming that this nearly-transparent color is OK as a background-color because it'll only be used for backgrounds (and presumably we're assuming that, as a background, it'll be layered on top of some other background-color that provides sufficient contrast). (B) The translations code is assuming that it can swap the background/foreground colors here and still have reasonable contrast (note in the first code-snippet in comment 4, we've got a `fill` color-variable as our background, and a `background` color-variable as our foreground). These assumptions are in conflict, which is what's causing this issue.