Once you paused your app, open both the windows.
In the thread window, you can select an entry by mouse, and then must use the tool-button "current" to make it active.
If you press "current" the stack will likely change.
On the stack, you can double click, which should take you to the source code (if a line and source file are shown).
You can use the tool-button "green arrow" / "->" (or "current" depending on Lazarus version) to activate the stack frame.
If you "activate" a stack frame locals, watches and value hints will be shown according to that stack frame.
If your top frame has no source, but you have sources maybe for the 2nd or 3rd (or further down) frame, then you can then set breakpoints in that code (after the line that is highlighted when double clicking).
You can then run to that breakpoint, and single step from there.
If you do not reach that breakpoint, then your app may hang in the called code (upper frames).