Custom App (Static Tab) with SharePoint URL stays blank on first load in Teams Desktop Client

Hello hyunjun,

Could you please try adding following in your App Manifest as workaround and let us know if you are facing any issue?

"staticTabs": [
    {
      "entityId": "TestTabId",
      "name": "Test Tab",
      "contentUrl": "https://microsoft.sharepoint.com/teams/MyTestSite/_layouts/15/teamslogon.aspx?spfx=true&dest=/teams/MyTestSite/SitePages/Test.aspx",
      "scopes": ["personal"],
      "websiteUrl": "https://fullwebsite.website.com/about"
    }
  ],
  "validDomains": [
    "*.login.microsoftonline.com",
    "microsoft.sharepoint.com",
    "microsoft.sharepoint-df.com",
    "spoppe-a.akamaihd.net",
    "spoprod-a.akamaihd.net",
    "resourceseng.blob.core.windows.net",
    "msft.spoppe.com"
  ],
  "webApplicationInfo": {
    "resource": "https://microsoft.sharepoint.com",
    "id": "00000003-0000-0ff1-ce00-000000000000"
  }