Redraw problems with lagless vsync enabled

[johnnovak]

I can trigger this visual glitch reliably in one specific part of Shadow of the Beast and only with Lagless Vsync enabled. The issue is not present with normal vsync or no vsync.

Tested on 5.3.0 only. I was running my monitor in fixed 50 Hz mode for this test.

Repro steps

I'm using SPS 1357 and the below config:

Shadow of the Beast.zip

Paste the following into your winuae.ini:

[FilterPresets]
Laced PAL 3.0x=0,1,4000.000000,1000.000000,1.000000,1.000000,0.000000,0.000000,0,0,17,1,0,0,2,2,0,0,0,0,0,0,0,0,0,1,0,0,-1,0,-1,0
Laced PAL 3.5x=0,1,5000.000000,1500.000000,1.000000,1.000000,0.000000,0.000000,0,0,17,1,0,0,2,2,0,0,0,0,0,0,0,0,0,1,0,0,-1,0,-1,0
PAL 3.0x=0,1,1000.000000,1000.000000,1.000000,1.000000,0.000000,0.000000,0,0,17,1,1,1,2,2,0,0,0,0,0,0,0,0,0,1,0,0,-1,0,-1,0
PAL 3.5x=0,1,1500.000000,1500.000000,1.000000,1.000000,0.000000,0.000000,0,0,17,1,1,1,2,2,0,0,0,0,0,0,0,0,0,1,0,0,-1,0,-1,0

1. Start the game, then start going right until you're at the place with the spikey things coming out of the ground (can't trigger this at the start position).
2. F12, Host/Filter tab, select the PAL 3.5x preset, and hit Load. Click OK to close the config dialog.
3. The blue rectangle should show up. If it doesn't show up at the first attempt, try pressing F12 and Enter a few times.
4. You'll notice the blue rectangle will be more or less flip-flopping between shown/not shown as you keep pressing F12 and Enter. It might sometimes also go away on its own after a few seconds.