I spotted the following error while running toolkit/components/extensions/test/browser/browser_ext_themes_ntp_colors_perwindow.js.
The test creates a new window, applies a theme to it, closes the window and then unloads the extension (relevant source). The error happens because the unload handler iterates over all keys in windowOverrides and then tries to detach the theme from the window. But as the error shows, the window isn't around any more. If you look at ext-theme.js, then it is obvious that the Theme is never cleaned up when a window is removed. This should be fixed.

Console message: [JavaScript Error: "Invalid window ID: 32" {file: "resource://gre/modules/ExtensionUtils.jsm" line: 54}]
ExtensionError@resource://gre/modules/ExtensionUtils.jsm:54:49
getWindow@chrome://extensions/content/parent/ext-tabs-base.js:1535:13
unload@chrome://extensions/content/parent/ext-theme.js:392:23
onShutdown@chrome://extensions/content/parent/ext-theme.js:428:15
ExtensionAPI/<@resource://gre/modules/ExtensionCommon.jsm:356:14
wrapper@resource://gre/modules/ExtensionCommon.jsm:300:14
emit@resource://gre/modules/ExtensionCommon.jsm:327:32
emit@resource://gre/modules/Extension.jsm:1752:25
shutdown@resource://gre/modules/Extension.jsm:2378:10
receiveMessage@resource://specialpowers/SpecialPowersParent.jsm:1048:26

This bug in themes code results in memory leaks. Do you know who can work on this?

I don't actively have time to work on this, sorry.

needinfo for https://phabricator.services.mozilla.com/D47663#1526885

I'll take over. Thanks for the initial patch.

(not a P1, and I'm not actively working on this because I'm mentoring a couple of bugs related to the theme API, and don't want to stomp on that with this change. Will revisit this bug later)