Start the panel slide after the window is on screen

[evowhale]

Problem

Found while testing 1.3.1 before release: closing the panel slides down correctly, but opening it pops in. The motion is asymmetric even though showPanel and hidePanel animate the same view the same way.

Cause

#22 moved the reveal from animating the window to sliding the content inside a fixed panel frame. showPanel still started that animation in the same turn as orderFrontRegardless(). Ordering a window in is not instant, the window server needs a composited frame before anything reaches the screen.

Instrumented a release build and logged timestamps across six open/close cycles:

ordered-in -> anim-start :  62, 51, 48, 60, 47, 53 ms   (mean 53)
anim-start -> anim-end   : 278, 261, 257, 261, 259, 263 ms  (duration is 250)
y at anim-start          : -280.0 every time

The slide is 250ms easeOut. A 53ms head start is 20% of the duration and roughly 36% of the distance on that curve, so the panel became visible already a third of the way up. hidePanel is unaffected because its window is already on screen when it animates.

Fix

• Push the parked first frame to the screen with displayIfNeeded() and a CATransaction.flush() right after ordering the window in.
• Start the slide on the next main actor turn, so the whole easeOut curve happens while the panel is visible.
• Drop the window shadow before orderFrontRegardless() rather than after, so the first composited frame never draws a shadow around empty space.

Cost

Content starts moving about 50ms later than before. Hotkey to fully open is about 310ms. Still far better than the 168ms of panel construction this path paid before #18.

Testing

Release build (Developer ID, notarized), installed to /Applications, single display:

• Open animation rides up the full distance, matches the close animation
• Close animation unchanged
• Quick Look preview
• Panel resize as the visible clip count changes
• Pinboard tab has no horizontal scrollbar (Hide the pinboard scroll indicator #23)
• Dual display, secondary stacked below primary, still opens on the screen under the pointer ([Bug]: History appears on second screen in dual screen setups #20)