Thank you @Timo_Kuisma1. But like I mentioned in my post: I did already try to apply your solution via the Katalon project settings > Desired Capabilities for Chrome (headless). See my supplied screenshot above.
I can see below settings in my settings\internal com.kms.katalon.core.webui.chrome (headless)
{"HEADLESS_DRIVER":{"prefs":{"download.prompt_for_download":false,"download.directory_upgrade":true,"safebrowsing.enabled":false,"safebrowsing.disable_download_protection":true,"download_dir":"C:\\KatalonDownloads","profile.default_content_settings.popups":"0","download.default_directory":"C:\\KatalonDownloads","download":{"default_directory":"C:\\KatalonDownloads"}},"excludeSwitches":["enable-automation"],"args":["--disable-infobars"]}}
I tried also “profile.default_content_settings.popups” as a number which is then set to 0.0 by Katalon. Doesn’t seem to download it. The click action was successful, I left enough time to allow to finish the download,… Works on Chrome, not on headless Chrome. 
@Russ_Thomas : Thanks for your reply. I did however already read your handy topic, but again, it works fine on normal Chrome (with the below settings), but it’s the Chrome Headless where it doesn’t.
{"CHROME_DRIVER":{"excludeSwitches":["enable-automation"],"useAutomationExtension":false,"prefs":{"download.default_directory":"C:\\KatalonDownloads"}}}
As a workaround, I tried to get it working for Firefox. But there I get annoyed by the save/open dialog which I can’t seem to get through. I supplied following desired capabilities (like suggested in topic Automatic download on firefox - #7 by paulo.resende and here Set Firefox profile to download files automatically using Selenium and Java - Stack Overflow ), but my profile does NOT seem to be applied when running in Katalon. Can’t figure out why… These are my firefox desired capabilites settings:
{"FIREFOX_DRIVER":{"firefox_binary":"C:\\Users\\ABC\\AppData\\Local\\Mozilla Firefox\\firefox.exe","firefox_profile":{"browser.download.folderList":2.0,"browser.helperApps.alwaysAsk.force":false,"browser.download.manager.showWhenStarting":false,"browser.download.dir":"C:\\KatalonDownloads","browser.download.downloadDir":"C:\\KatalonDownloads","browser.download.defaultFolder":"C:\\KatalonDownloads","browser.helperApps.neverAsk.saveToDisk":"application/pdf;charset\u003dUTF-8","security.insecure_field_warning.contextual.enabled":false,"browser.helperApps.neverAsk.openFile":"application/pdf;charset\u003dUTF-8","marionette":true,"pdfjs.disabled":true}}}
When I debug and click manually on the download button in the firefox popup I notice it doesn’t even get downloaded in my expected download.dir C:\KatalonDownloads. So I suspect my profile to not be applied for Firefox. Also here, no idea why. 
This is the log message in the Console:
2020-05-20 14:40:48.931 INFO c.k.katalon.core.main.TestCaseExecutor - START Test Cases/Projects/XYZ/Test/Functional/downloadFullRequest
2020-05-20 14:40:49.803 INFO c.k.katalon.core.main.TestCaseExecutor - (Default) downloadRequestType = fullRequest
2020-05-20 14:40:49.952 INFO com.kms.katalon.core.util.KeywordUtil - testCaseContext.getTextCaseId()=downloadFullRequest
2020-05-20 14:40:51.253 INFO c.k.k.core.webui.driver.DriverFactory - Starting ‘Firefox’ driver
mei 20, 2020 2:40:51 PM org.openqa.selenium.remote.DesiredCapabilities firefox
INFO: Using new FirefoxOptions() is preferred to DesiredCapabilities.firefox()
2020-05-20 14:40:51.591 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set preference: [‘firefox_binary’, ‘C:\Users\ABC\AppData\Local\Mozilla Firefox\firefox.exe’]
2020-05-20 14:40:51.592 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.download.folderList’, ‘2’]
2020-05-20 14:40:51.594 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.helperApps.alwaysAsk.force’, ‘false’]
2020-05-20 14:40:51.595 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.download.manager.showWhenStarting’, ‘false’]
2020-05-20 14:40:51.595 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.download.dir’, ‘C:\KatalonDownloads’]
2020-05-20 14:40:51.618 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.download.downloadDir’, ‘C:\KatalonDownloads’]
2020-05-20 14:40:51.621 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.download.defaultFolder’, ‘C:\KatalonDownloads’]
2020-05-20 14:40:51.623 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.helperApps.neverAsk.saveToDisk’, ‘application/pdf;charset=UTF-8’]
2020-05-20 14:40:51.625 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘security.insecure_field_warning.contextual.enabled’, ‘false’]
2020-05-20 14:40:51.626 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘browser.helperApps.neverAsk.openFile’, ‘application/pdf;charset=UTF-8’]
2020-05-20 14:40:51.627 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘marionette’, ‘true’]
2020-05-20 14:40:51.628 INFO c.k.k.c.w.util.WebDriverPropertyUtil - User set Firefox profile preference: [‘pdfjs.disabled’, ‘true’]
2020-05-20 14:40:51.630 INFO c.k.k.core.webui.driver.DriverFactory - Action delay is set to 0 milliseconds
1589978462305 mozrunner::runner INFO Running command: “C:/Users/ABC/AppData/Local/Mozilla Firefox/firefox.exe” “-marionette” “-foreground” “-no-remote” “-profile” “C:\Users\ABC~1\AppData\Local\Temp\rust_mozprofile.KwcXF9b8qprj”
1589978463412 addons.webextension. WARN Loading extension ‘null’: Reading manifest: Warning processing permissions: Error processing permissions.12: Value “debugger” must either: must either [must either [be one of [“clipboardRead”, “clipboardWrite”, “geolocation”, “idle”, “notifications”], be one of [“bookmarks”], be one of [“find”], be one of [“history”], be one of [“menus.overrideContext”], be one of [“search”], be one of [“topSites”], be one of [“activeTab”, “tabs”, “tabHide”], be one of [“browserSettings”], be one of [“cookies”], be one of [“downloads”, “downloads.open”], be one of [“privacy”], be one of [“webNavigation”], or be one of [“webRequest”, “webRequestBlocking”]], be one of [“alarms”, “mozillaAddons”, “storage”, “unlimitedStorage”], be one of [“browsingData”], be one of [“captivePortal”], be one of [“devtools”], be one of [“identity”], be one of [“menus”, “contextMenus”], be one of [“normandyAddonStudy”], be one of [“pkcs11”], be one of [“sessions”], be one of [“urlbar”], be one of [“geckoProfiler”], be one of [“contextualIdentities”], be one of [“dns”], be one of [“activityLog”], be one of [“management”], be one of [“networkStatus”], be one of [“proxy”], be one of [“nativeMessaging”], be one of [“telemetry”], be one of [“theme”], or match the pattern /^experiments(.\w+)+$/], or must either [be one of [“<all_urls>”], must either [match the pattern /^(https?|wss?|file|ftp|*)://(*|*.[^/]+|[^/]+)/.$/, or match the pattern /^file:///.$/], or match the pattern /^resource://(*|*.[^/]+|[^/]+)/.$|^about:/]
1589978463413 addons.webextension. WARN Loading extension ‘null’: Reading manifest: Warning processing offline_enabled: An unexpected property was found in the WebExtension manifest.
1589978463534 addons.webextension.doh-rollout@mozilla.org WARN Loading extension ‘doh-rollout@mozilla.org’: Reading manifest: Invalid extension permission: networkStatus
1589978463582 addons.webextension.screenshots@mozilla.org WARN Loading extension ‘screenshots@mozilla.org’: Reading manifest: Invalid extension permission: mozillaAddons
1589978463582 addons.webextension.screenshots@mozilla.org WARN Loading extension ‘screenshots@mozilla.org’: Reading manifest: Invalid extension permission: telemetry
1589978463583 addons.webextension.screenshots@mozilla.org WARN Loading extension ‘screenshots@mozilla.org’: Reading manifest: Invalid extension permission: resource://pdf.js/
1589978463583 addons.webextension.screenshots@mozilla.org WARN Loading extension ‘screenshots@mozilla.org’: Reading manifest: Invalid extension permission: about:reader
1589978465021 addons.webextension.{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a} WARN Loading extension ‘{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a}’: Reading manifest: Warning processing permissions: Error processing permissions.12: Value “debugger” must either: must either [must either [be one of [“clipboardRead”, “clipboardWrite”, “geolocation”, “idle”, “notifications”], be one of [“bookmarks”], be one of [“find”], be one of [“history”], be one of [“menus.overrideContext”], be one of [“search”], be one of [“topSites”], be one of [“activeTab”, “tabs”, “tabHide”], be one of [“browserSettings”], be one of [“cookies”], be one of [“downloads”, “downloads.open”], be one of [“privacy”], be one of [“webNavigation”], or be one of [“webRequest”, “webRequestBlocking”]], be one of [“alarms”, “mozillaAddons”, “storage”, “unlimitedStorage”], be one of [“browsingData”], be one of [“captivePortal”], be one of [“devtools”], be one of [“identity”], be one of [“menus”, “contextMenus”], be one of [“normandyAddonStudy”], be one of [“pkcs11”], be one of [“sessions”], be one of [“urlbar”], be one of [“geckoProfiler”], be one of [“contextualIdentities”], be one of [“dns”], be one of [“activityLog”], be one of [“management”], be one of [“networkStatus”], be one of [“proxy”], be one of [“nativeMessaging”], be one of [“telemetry”], be one of [“theme”], or match the pattern /^experiments(.\w+)+$/], or must either [be one of [“<all_urls>”], must either [match the pattern /^(https?|wss?|file|ftp|*)://(*|*.[^/]+|[^/]+)/.$/, or match the pattern /^file:///.$/], or match the pattern /^resource://(*|*.[^/]+|[^/]+)/.$|^about:/]
1589978465021 addons.webextension.{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a} WARN Loading extension ‘{54dcd9e9-903f-43e9-95f0-22bd0e0f3c0a}’: Reading manifest: Warning processing offline_enabled: An unexpected property was found in the WebExtension manifest.
JavaScript error: resource://gre/modules/XULStore.jsm, line 66: Error: Can’t find profile directory.
1589978469736 Marionette INFO Listening on port 56800
1589978470263 Marionette WARN TLS certificate errors will be ignored for this session
1589978470265 Marionette INFO Proxy settings initialised: {“proxyType”:“direct”}
mei 20, 2020 2:41:10 PM org.openqa.selenium.remote.ProtocolHandshake createSession
INFO: Detected dialect: W3C
2020-05-20 14:41:10.658 INFO c.k.k.core.webui.driver.DriverFactory - sessionId = e8a00148-0a6c-4e7a-b22e-f327c6710143
2020-05-20 14:41:10.695 INFO c.k.k.core.webui.driver.DriverFactory - browser = Firefox 76.0
2020-05-20 14:41:10.696 INFO c.k.k.core.webui.driver.DriverFactory - platform = Windows 10
2020-05-20 14:41:10.697 INFO c.k.k.core.webui.driver.DriverFactory - seleniumVersion = 3.141.59
2020-05-20 14:41:10.698 INFO c.k.k.core.webui.driver.DriverFactory - proxyInformation = ProxyInformation{proxyOption=NO_PROXY, proxyServerType=HTTP, username=, password=*****, proxyServerAddress=, proxyServerPort=0, exceptionList=}
JavaScript error: resource://gre/modules/NetworkGeolocationProvider.jsm, line 555: TypeError: xhr.response.location is undefined
It does show a bunch of warnings and at the end a JavaScript error, but don’t think those are related? After that my tests loads fine up until where I expect my download to be automatically downloaded. Yet I’m blocked by that firefox popup
I tried to go and check that temp profile mentioned in the console log
C:\Users\ABC~1\AppData\Local\Temp\rust_mozprofile.KwcXF9b8qprj and indeed, I do not retrieve the prefs settings there in the prefs.js file. Might that be a clue my custom profile is not applied?
Sorry for the long post, but I tried to be as detailed as possible. Any help here would be greatly appreciated!!!
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