fix(memory): empty-bank graph placeholder no longer locks up the dashboard

[thinmintdev]

Problem

In the Memory tab, selecting a bank with no graph data (or any empty search/filter result) made the entire dashboard unclickable — it appeared to "lock up." Reported: "interacting with the memory map while a bank with no data is selected causes the browser to lock up. Search, graph view etc — whenever it's empty it locks."

Root cause (CSS, not a JS infinite loop)

The .mem-graph-empty placeholder carried position: absolute; inset: 0 — a leftover from when it overlaid the old position: relative .mem-graph-stage. The A/B/C graph overhaul (#752) moved the placeholder into .mg-host, which is position: static. With no positioned ancestor, the absolute box resolved its containing block to the viewport: it rendered full-page and transparent, intercepting every pointer event, while .mg-host collapsed to height: 0.

Verified live against the empty private__claude-dev bank on CT105 (Playwright): the empty box measured {0,0, 1266×812} and elementFromPoint over the bank picker and direction switch returned the overlay, not the controls. Injecting the fix CSS restored hit-testing and clicks immediately.

Fix

• .mem-graph-empty is now in-flow (min-height instead of absolute; inset:0) — it fills the stage area without escaping or collapsing the host.
• .mg-host gets position: relative so any future absolute stage child stays contained to the graph area (defense-in-depth).

Tests

• New e2e spec memory-graph-empty-v3 selects the empty bank and asserts the placeholder does not overlap the toolbar and controls stay hit-testable. Fails without the fix, passes with it.
• Added an empty bank to the forced-mock dataset (the empty path had zero coverage — that's why this slipped through); updated the bank-count assertion (5 → 6).
• Full memory e2e suite green; production build clean.

Deploy note

CT105 runtime is currently on another session's branch (fix/npu-flm-alias-routing, diverged) — do not deploy this over that in-progress work. Deploy once CT105 is back on main.

Follow-ups (not in this PR)

• Directions B/Structured & C/Ego gate their empty shell on the fact nodeCount even when entity data exists, so a fact-empty bank with entities shows "No graph data" instead of the co-occurrence matrix.
• GraphEgo's onCenter → setEgoCenter → ego refetch → defaultCenter chain can refetch-loop on large banks if the slice's highest-degree node differs from the requested center.

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