Why does my side-loaded Excel Javascript add-in open in the external browser, not the task pane?

Question

ETA: Note: I have just noticed that before the external browser opens taskpane.html, some process asks to save that file on my local drive. I was too flustered to notice this last week, apparently.

I am developing a Javascript (really Typescript) api add-in for Excel. It seems to be working correctly on my local machine. When I upload the source to a server and side-load the manifest, the new button to open a task pane appears on the Ribbon, but when I press the button, the html file opens in an external browser, not the task pane.

Steps to reproduce (on my system):

1. Set up a development environment as described here. (In short: install Node.js, npm, yeoman, and Visual Studio Code.)
2. Generate a sample add-in with yeoman as described here. (In short: use the command "yo office", select Office Add-in Task Pane project, script type Typescript, Name it Anything, support Excel.)
3. Publish this add-in as described here. (in short: run "npm run build", upload the generated dist folder to a server, edit the manifest to replace localhost:3000 with the URL of the server)
4. Side-load the add-in as described here.
5. Try to run the add-in and see the taskpane.html file open in your external browser.

ETA:

Here's the manifest I'm currently using. (Except "MyCompanyName" is not the URL)

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<OfficeApp xmlns="http://schemas.microsoft.com/office/appforoffice/1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0" xmlns:ov="http://schemas.microsoft.com/office/taskpaneappversionoverrides" xsi:type="TaskPaneApp">
  <Id>27a7b548-9df8-4f2c-8554-0f4e9ebe4d6a</Id>
  <Version>1.0.0.0</Version>
  <ProviderName>Contoso</ProviderName>
  <DefaultLocale>en-US</DefaultLocale>
  <DisplayName DefaultValue="SIPmath"/>
  <Description DefaultValue="A template to get started."/>
  <IconUrl DefaultValue="https://MyCompanyName.squarespace.com/s/icon-32.png"/>
  <HighResolutionIconUrl DefaultValue="https://MyCompanyName.squarespace.com/s/icon-80.png"/>
  <SupportUrl DefaultValue="https://www.contoso.com/help"/>
  <AppDomains>
    <AppDomain>contoso.com</AppDomain>
  </AppDomains>
  <Hosts>
    <Host Name="Workbook"/>
  </Hosts>
  <DefaultSettings>
    <SourceLocation DefaultValue="https://MyCompanyName.squarespace.com/s/taskpane.html"/>
  </DefaultSettings>
  <Permissions>ReadWriteDocument</Permissions>
  <VersionOverrides xmlns="http://schemas.microsoft.com/office/taskpaneappversionoverrides" xsi:type="VersionOverridesV1_0">
    <Hosts>
      <Host xsi:type="Workbook">
        <DesktopFormFactor>
          <GetStarted>
            <Title resid="GetStarted.Title"/>
            <Description resid="GetStarted.Description"/>
            <LearnMoreUrl resid="GetStarted.LearnMoreUrl"/>
          </GetStarted>
          <FunctionFile resid="Commands.Url"/>
          <ExtensionPoint xsi:type="PrimaryCommandSurface">
            <OfficeTab id="TabHome">
              <Group id="CommandsGroup">
                <Label resid="CommandsGroup.Label"/>
                <Icon>
                  <bt:Image size="16" resid="Icon.16x16"/>
                  <bt:Image size="32" resid="Icon.32x32"/>
                  <bt:Image size="80" resid="Icon.80x80"/>
                </Icon>
                <Control xsi:type="Button" id="TaskpaneButton">
                  <Label resid="TaskpaneButton.Label"/>
                  <Supertip>
                    <Title resid="TaskpaneButton.Label"/>
                    <Description resid="TaskpaneButton.Tooltip"/>
                  </Supertip>
                  <Icon>
                    <bt:Image size="16" resid="Icon.16x16"/>
                    <bt:Image size="32" resid="Icon.32x32"/>
                    <bt:Image size="80" resid="Icon.80x80"/>
                  </Icon>
                  <Action xsi:type="ShowTaskpane">
                    <TaskpaneId>ButtonId1</TaskpaneId>
                    <SourceLocation resid="Taskpane.Url"/>
                  </Action>
                </Control>
              </Group>
            </OfficeTab>
          </ExtensionPoint>
        </DesktopFormFactor>
      </Host>
    </Hosts>
    <Resources>
      <bt:Images>
        <bt:Image id="Icon.16x16" DefaultValue="https://MyCompanyName.squarespace.com/s/icon-16.png"/>
        <bt:Image id="Icon.32x32" DefaultValue="https://MyCompanyName.squarespace.com/s/icon-32.png"/>
        <bt:Image id="Icon.80x80" DefaultValue="https://MyCompanyName.squarespace.com/s/icon-80.png"/>
      </bt:Images>
      <bt:Urls>
        <bt:Url id="GetStarted.LearnMoreUrl" DefaultValue="https://go.microsoft.com/fwlink/?LinkId=276812"/>
        <bt:Url id="Commands.Url" DefaultValue="https://MyCompanyName.squarespace.com/s/commands.html"/>
        <bt:Url id="Taskpane.Url" DefaultValue="https://MyCompanyName.squarespace.com/s/taskpane.html"/>
      </bt:Urls>
      <bt:ShortStrings>
        <bt:String id="GetStarted.Title" DefaultValue="Get started with your sample add-in!"/>
        <bt:String id="CommandsGroup.Label" DefaultValue="Commands Group"/>
        <bt:String id="TaskpaneButton.Label" DefaultValue="Show Taskpane"/>
      </bt:ShortStrings>
      <bt:LongStrings>
        <bt:String id="GetStarted.Description" DefaultValue="Your sample add-in loaded succesfully. Go to the HOME tab and click the 'Show Taskpane' button to get started."/>
        <bt:String id="TaskpaneButton.Tooltip" DefaultValue="Click to Show a Taskpane"/>
      </bt:LongStrings>
    </Resources>
  </VersionOverrides>
</OfficeApp>

Answer 1

Turns out squarespace is the problem. I uploaded the add-in to a different URL and it works... well, not 'fine', because there are still bugs in it, but at least it works!