The grammar pattern たなり(たなり / なり) expresses that after an action was completed, the resulting state remains unchanged – often implying that something expected to follow never happened. It conveys the meaning of "ever since doing ~," "having done ~ and then staying that way," or "leaving something as is."
The basic formation is Verb[た] + なり, though the form Verb[た] + なり + Verb[た] + なり should not be confused with this pattern (that is a separate "doing X and Y alternately" construction).
The most common use describes someone who did something and then never reversed or followed up on that action. There is often a nuance of complaint, concern, or surprise that the situation persists.
It can also describe leaving an object or situation as is – the subject did something and left the result unchanged, often with a critical or disapproving tone.
たなり is used to describe someone remaining in a physical posture – sitting, lying down, standing – and doing something else or doing nothing at all.
This is a literary / formal expression most commonly found in written Japanese or formal spoken contexts. It carries a strong implication that the continuation of the state is undesirable, unexpected, or noteworthy. In casual conversation, similar ideas would more likely be expressed with 〜~たまま or 〜~っぱなし.
Compared to 〜~たまま, たなり places stronger emphasis on the fact that the expected follow-up action did not occur, and it often has a more dramatic or emotional tone.