SkinxTest.testJavascriptExtension is flickering

Error Message

Expected condition failed: waiting for org.xwiki.test.ui.XWikiWebDriver$$Lambda$456/1830231264@5d5d6f81 (tried for 10 second(s) with 500 milliseconds interval)
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:17:03'
System info: host: '485b26252d7f', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.19.0-9-amd64', java.version: '1.8.0_282'
Driver info: org.xwiki.test.ui.XWikiWebDriver
Capabilities {acceptInsecureCerts: true, browserName: firefox, browserVersion: 74.0, javascriptEnabled: true, moz:accessibilityChecks: false, moz:buildID: 20200309095159, moz:geckodriverVersion: 0.26.0, moz:headless: false, moz:processID: 1217, moz:profile: /tmp/rust_mozprofilejlUoGk, moz:shutdownTimeout: 60000, moz:useNonSpecCompliantPointerOrigin: false, moz:webdriverClick: true, pageLoadStrategy: normal, platform: LINUX, platformName: LINUX, platformVersion: 4.19.0-9-amd64, rotatable: false, setWindowRect: true, strictFileInteractability: false, timeouts: {implicit: 0, pageLoad: 300000, script: 30000}, unhandledPromptBehavior: dismiss and notify}
Session ID: 7592a4e9-8f11-43dd-9634-8e87f93f9c44

Stacktrace

org.openqa.selenium.TimeoutException: 
Expected condition failed: waiting for org.xwiki.test.ui.XWikiWebDriver$$Lambda$456/1830231264@5d5d6f81 (tried for 10 second(s) with 500 milliseconds interval)
Build info: version: '3.141.59', revision: 'e82be7d358', time: '2018-11-14T08:17:03'
System info: host: '485b26252d7f', ip: '172.17.0.2', os.name: 'Linux', os.arch: 'amd64', os.version: '4.19.0-9-amd64', java.version: '1.8.0_282'
Driver info: org.xwiki.test.ui.XWikiWebDriver
Capabilities {acceptInsecureCerts: true, browserName: firefox, browserVersion: 74.0, javascriptEnabled: true, moz:accessibilityChecks: false, moz:buildID: 20200309095159, moz:geckodriverVersion: 0.26.0, moz:headless: false, moz:processID: 1217, moz:profile: /tmp/rust_mozprofilejlUoGk, moz:shutdownTimeout: 60000, moz:useNonSpecCompliantPointerOrigin: false, moz:webdriverClick: true, pageLoadStrategy: normal, platform: LINUX, platformName: LINUX, platformVersion: 4.19.0-9-amd64, rotatable: false, setWindowRect: true, strictFileInteractability: false, timeouts: {implicit: 0, pageLoad: 300000, script: 30000}, unhandledPromptBehavior: dismiss and notify}
Session ID: 7592a4e9-8f11-43dd-9634-8e87f93f9c44
	at org.openqa.selenium.support.ui.WebDriverWait.timeoutException(WebDriverWait.java:95)
	at org.openqa.selenium.support.ui.FluentWait.until(FluentWait.java:272)
	at org.xwiki.test.ui.XWikiWebDriver.waitUntilCondition(XWikiWebDriver.java:226)
	at org.xwiki.test.ui.XWikiWebDriver.waitUntilJavascriptCondition(XWikiWebDriver.java:597)
	at org.xwiki.test.ui.po.editor.EditPage.waitUntilPageJSIsLoaded(EditPage.java:292)
	at org.xwiki.test.ui.po.BasePage.<init>(BasePage.java:125)
	at org.xwiki.test.ui.po.editor.EditPage.<init>(EditPage.java:47)
	at org.xwiki.test.ui.po.editor.ObjectEditPage.<init>(ObjectEditPage.java:41)
	at org.xwiki.test.ui.po.editor.ObjectEditPage.gotoPage(ObjectEditPage.java:55)
	at org.xwiki.test.ui.SkinxTest.testJavascriptExtension(SkinxTest.java:76)