OK I have come back to this and found a solution that is a bit complex but seems to be really reliable, and follows standards to at least some extent.

Window instance variables:

Boolean ib_navigate_called // have we called navigate on the source file?
Boolean ib_navigate_started // has it really started navigating, after we called it?

Script to start navigation:

ib_navigate_called = FALSE
ib_navigate_started = FALSE

if wb_1.navigate(as_source) <> 1 then
    // give error message
    return
end if

ib_navigate_called = TRUE // indicate we have tried to start navigating

WebBrowser control NavigationStart event:

// We need to know it has really started navigating before we try to determine whether it has finished
if ib_navigate_called then ib_navigate_started = TRUE

WebBrowser control NavigationProgressIndex event:

// Navigation has been started. If it indicates 100 percent finished, check the document.readyState.
// If that is "complete", print the PDF file.

string lsRet

if NOT ib_navigate_started then return

if progressindex < 100 then return

EvaluateJavaScriptSync("document.readyState", REF lsRet)
if Pos(lsRet, '"complete"') = 0 then
    return
end if
ib_navigate_started = FALSE // prevent recursion & thus doing any following steps twice

// now do whatever you need to do, it should be fully loaded!

When developing a test browser using the new browser control (https://powertothebuilder.blogspot.com/2020/08/develop-web-browser-in-powerbuilder.html), I found that the NavigationStateChanged event was the best event to use. I display a spinning circle in the tab when the page is loading. In the NavigationStateChanged event, I turn the spinner off. Seems to work well.

I hope that helps.

Having stared at the WebBrowser docs some more I'm really concerned that there may currently be no solution to this.

With the Microsoft WebBrowser control that we already use (as an HTML editor in DesignMode, as well as for viewing mail-merge results), we use the DocumentComplete event (which the WebBrowser docs say it doesn't have an equivalent for) to determine whether a navigation is completed. Within that event script, we also check the Busy and ReadyState properties (which again the WebBrowser docs say it has no equivalents for).

I don't suppose the Other event might help, to catch some Chromium event that PowerBuilder's control isn't explicitly providing an interface to?

I see that there's a GetSource() method, which I suppose you could use to see whether the entire source has been loaded (if you had also read the HTML some other way, and could compare that to the results of GetSource()), but that wouldn't tell you for instance whether any images had been loaded. And for pages with frames, GetSource() returns only the text of the main frame.

I've tried to use the "downloadstatuschanged" event to know whether a pdf has fully downloaded and ... there's a bug: The event is fired multiple times with the parameter "percent" having a value of "100".
I've reported it as a (private) bug yesterday and Appeon is going to deal with it.

That makes the webbrowser control unfit for use for what we want to do with it. I'll just wait.

Hi Dan,

For the navigation completed issue, CEF doesn’t provide a conventional interface to load the page completely. So technically it is not supported.

Please use the navigationprogressindex event to determine the page loading process, you can consider it is complete when it returns 100.

For the GetSource issue, we will record it (GetSource() returns the text of all frames) as a requirement in our CR pool, I will get back to you once we have a plan to support this feature.

Regards,
ZhaoKai

Having read Dan's question and all responses and seeing that the page itself's load event has been mentioned several times I would like to share my code to test if it has been fully loaded:

function boolean uf_is_page_loaded()
//iwb is a reference to the current webbrowser

string ls_script, ls_result 

ib_processing = true
ls_script = '(function(){return (document.readyState === "complete");})();'

if iwb.evaluatejavascriptSync(ls_script, ls_result) = 1 then
ls_result = uf_get_parsed_json(ls_result)
if ls_result = "true" then
return true
end if
end if

return false

function string uf_get_parsed_json(string as_json_value)

string ls_result
JsonParser lnv_JsonParser
Long ll_RootObject

lnv_JsonParser = Create JsonParser
lnv_JsonParser.LoadString(as_json_value)
ll_RootObject = lnv_JsonParser.GetRootItem()
ls_result = lnv_JsonParser.GetItemString(ll_RootObject, "value")
destroy lnv_JsonParser

return ls_result

I wasn't aware of the navigationprogressindex event. Is checking it effectively the same as my uf_is_page_loaded and which is prefferable?