Problem resolved by support team from Microsoft Developer Support for Internet Explorer.
The frame/dialog targeting problem described above is the result of a change introduced with installation of IE11 on a computer, affecting behavior of User Account Control: Use Admin Approval Mode for the built-in Administrator account setting within local security policy.
This setting can be configured using the Local Security Policy snap-in, in Local Policies / Security Options branch, under Policy item called: “User Account Control: Use Admin Approval Mode for the built-in Administrator account”.
In simple terms, the steps needed to correct the problem are:
Before installation of IE11 – no need to do anything on any Windows or IE version to get frame targeting to work under default Admin. It works even with “User Account Control: Use Admin Approval Mode …” set to Disabled.
After installation of IE11 – need to Enable “User Account Control: Use Admin Approval Mode for the built-in Administrator account” to get frame/dialog targeting to work for default Admin.
Confirmed the solution works with IE11 on Windows 2012R2 as well as Windows 7 SPK1, the adjustment takes effect after a restart. There is no need to make any changes to HTML pages.