Light theme flashes for a second before the page loads in dev build

[uBlock-user]

Prerequisites

Unless you have set your OS theme set to dark, you may not be able to reproduce it.

Description

As per the title.

Steps to Reproduce

• On the latest dev build, open Settings pane via the dashboard and set Theme to Dark

• Switch to different panes like My rules/My filters/Trusted sites, basically at every new page/pane, Light theme will flash again.

Expected behavior:

The Light theme which is not set shouldn't flash before the page loads

Actual behavior:

The Light theme which is not set flashes before the page loads

Your environment

• uBlock Origin version: 1.40.9b8
• Browser Name and version: Firefox
• Operating System and version: Win 10

[uBlock-user]

@gorhill So can this be addressed, now that you brought dark theme out of experimental status ?

[gorhill]

Looking into this.

[uBlock-user]

However this works only for
when prefers-color-scheme is properly set by the browser, and only
when uBO's theme selection is "auto", or when it happens to
match that of prefers-color-scheme.

So the opposite effect will still occur when I set the theme to Light and will face dark flashes ?

[gorhill]

If your setting in uBO is opposite of how your browser set prefers-color-scheme, there will be a flash, there is nothing which can be done to avoid this. If you use Auto in uBO, there should be no flash of background color (except on Linux, this is an open issue in Chromium).

• Browser prefers-color-scheme: light, uBO Auto or Light, no flash
• Browser prefers-color-scheme: dark, uBO Auto or Dark, no flash
• Browser prefers-color-scheme: light, uBO Dark, flash of white
• Browser prefers-color-scheme: dark, uBO Light, flash of black

[uBlock-user]

If you were not to deploy prefers-color-scheme would this issue occur ? Example - only options would be light and dark.

[gorhill]

to deploy prefers-color-scheme

uBO does not "deploy", it's a read-only value set by the browser, to give hint about the default background color. If uBO didn't read it, the flash of wrong background can still occur depending on whether uBO's default background color matches that of the browser. You get the same issue:

• Browser paint background white by default: uBO Dark, flash of white
• Browser paint background black by default: uBO Light, flash of black

The core issue is uBO does not know what is the default color used by the browser to paint the background. prefers-color-scheme merely helps uBO to make a good guess.