BackgroundWorker task blocks UI thread in my web application - asp.net

Question

Okay, I've been banging on this problem for close to 10 work hours now with no real progress to speak of. After going through multiple "solutions" offered on the web, I find myself still unable to accomplish what should be a really simple task -- pop up a modal dialog from my web application while I complete a long process I'm forcing the user to wait upon.
I've ripped out all the non-multi-threading-control aspects and am left with this bare bones structure and it still doesn't cut it:
In my ASPX file, I have two "please wait" displays, one is a DIV set up for use with bPopup, the other a DIV runat=server following the examples on BackgroundWorker. The bPopup dialog works great when used independently of any background work. But, when the background worker fires, the UI freezes until it completes, then both opens & closes the bPopup dialog as fast as it can, basically flashing the screen.
This is my code behind: I've cut it to the bare minimum to see a dialog pop up and go away again after the background task completes.
Protected Sub EditBtn_Click(sender As Object, e As EventArgs)
ScriptManager.RegisterStartupScript(Page, Page.GetType, Guid.NewGuid().ToString(), "javascript: bPopup = $('#element_to_pop_up').bPopup({ modal: true, modalClose: false});", True)
MenusBeingGenerated.Visible = True
bw = New BackgroundWorker()
bw.WorkerReportsProgress = True
bw.WorkerSupportsCancellation = True
bw.RunWorkerAsync()
End Sub
The javascript fires the bPopup, while MenusBeingGenerated is the name of the server-side DIV. The background task itself has been ripped down to just a do-nothing time-consumer.
Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles bw.DoWork
For i = 1 To 40
Threading.Thread.Sleep(250)
bw.ReportProgress(i, "Running...")
If bw.CancellationPending Then
bw.ReportProgress(i, "Cancelling...")
Exit For
End If
Next
' Cleanup
If bw.CancellationPending Then
e.Cancel = True
bw.ReportProgress(100, "Cancelled.")
End If
End Sub
Private Sub BackgroundWorker1_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles bw.ProgressChanged
StatusLabel.Text = e.ProgressPercentage.ToString & "% complete."
End Sub
Private Sub BackgroundWorker1_RunWorkerCompleted(sender As Object, e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles bw.RunWorkerCompleted
StatusLabel.Text = "Completed successfully"
ScriptManager.RegisterStartupScript(Page, Page.GetType, Guid.NewGuid().ToString(), "javascript: if (bPopup) { bPopup.close() };", True)
End Sub
When I run this code, and click the button that triggers it all there is a long pause then the screen blinks grey (the same behavior can be seen by opening and closing the bPopup from within the same button handler).
Adding breakpoints to walk through the code, I see the background task get launched via the button handler, it cycles through and the BackgroundWorker1_ProgressChanged handler gets called every quarter second, finally the BackgroundWorker1_RunWorkerCompleted method runs, and when that finishes the only page refresh finally happens (with the blink as bPopup fires & clears).
The DIV id=MenusBeingGenerated never displays, despite having been set as visible, nor does the label id=StatusLabel that resides within it ever get rendered.
Any clues what I've missed???
[ BTW, there are several questions along these lines on this and other web sites, none of the current answers apply. I've cookbooked this off several sites, thrown out multiple versions, and this is the closest I've gotten to something that works -- and it's no different than where I started: the user clicks the go button, a lot of nothing visible happens while the server churns out a new file, then the page refreshes when it's all done. ]