Operation by Instruction fails on macOS with multiple threads

Steps to reproduce:

• Open tests/manual/debugger/simple
• Set breakpoint in Thread::run
• uncomment testQThread();
• Start Debugging
• Turn on "Operate by instruction" once the breakpoint is hit

Expected:

• The Debugger switches to a assembler view

Actual

• Nothing seems to happen, only the current line cursor disappears

This is probably due to lldb's "break in multiple threads" feature.