Two independent root causes for '左右能拉，上下不能拉':
1. Event delivery: the top 6px edge band belongs to QMenuBar, which
actively consumes mouse events (menu hover) — nothing ever propagated
to QMainWindow, so T/TL/TR (and maximize-restore from top edge) were
dead on every platform. Install an event filter on menuBar()/statusBar():
within the edge band it handles hover cursor, press (grabMouse), drag
and release; outside the band everything passes through (menus open
normally, native QSizeGrip corner drag preserved).
2. Layout minimum: MonitorPanel's minimumSizeHint (~1015px of stacked
tables/group boxes) propagated through QTabWidget → central widget →
QMainWindow, making the window minimum size 898x1121. Any vertical
shrink was clamped dead. Set explicit per-tab-page minimumSize
(320x200) + window minimumSize (760x480).
Tests: test_resize.py now drives edges through real event delivery
(childAt → child widget, propagation/filter path) for L/R/T/B/TL/TR/BL,
verifies BR stays on QSizeGrip, maximize-restore via menubar filter,
and minimum-size regression. All 8 suites pass.
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