Fix right-panel gaps and jumps when resizing

[brsbl]

Human comments

What was wrong

The snap preview changed DOM flex independently of the panel library while both handled pointer input. Repeated drags could desynchronize panel width and open state, causing white strips, release jumps, and duplicate toggles.

What changed

• Dragging stays within both width limits, with the panel library owning every size update.
• Releasing the handle preserves its position without white strips or duplicate toggles.

How you verified

• Remote CI passed, including all 15 resize regression tests.
• Chrome for Testing 153.0.8010.36: limits, rapid reversals, viewport exit, keyboard resizing, reload, and reopening passed.
• All 21 recorded release frames retained the divider position without a gap.

Web screenshots use the same stopped thread and two drags to maximum width: 1720 × 1040 CSS pixels, captured at 2×.

Before — merge base cf51227e1	After — head d73c0e699

BB-Thread-ID: thr_3bp5in88fn

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