Existing Graphical User Interface (GUI) benchmarks predominantly rely on precise and stateless user instructions. In everyday usage, however, instructions are often \emph{vague and history-dependent}: users omit the exact details of their past operations because they no longer remember them, leaving the agent to recover the missing context from long and noisy interaction histories. To bridge this evaluation gap, we introduce \textbf{MemU-Bench}, a mobile benchmark of \textbf{118 vague tasks} grounded in 29 long-horizon user trajectories, where agents must distill raw interaction streams into memory and retrieve the missing context on demand, with task outcomes verified against serialized application databases. Alongside Success Rate, we incorporate Interaction Efficiency (IE) and Success weighted by Path Length (SPL) to expose agents that succeed by exhaustively searching across different applications rather than by \emph{genuinely recalling} the history. Evaluations show that without user history, state-of-the-art systems struggle under such ambiguity: the best-performing Gemini-3.1-Pro achieves an SPL of only 16.4\%, and end-to-end GUI agents fall below 5\%. To address this, we further propose \textbf{AppMem}, a training-free framework that distills user history into a \emph{structured, app-centric} memory bank, tracks interrupted intents across application switches, and selectively retrieves relevant memories via target-app inference. AppMem raises the SPL of GPT-5.5 from 15.7\% to 66.9\% (\textbf{+51.2\%}), outperforms a naive summary baseline by 9.3\%, and delivers consistent gains across evaluated backbones.

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