If you let "sprites5.bbc" run for a while, then interrupt with ESC, and then CHAIN "sprites2.bbc", it can crash

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This bug was reported by Lennart Benschop on 23/12/2023.
It shows the reboot message for Agon VPD 1.02 and then hangs

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I'm able to replicate this.
Note that it does not seem to happen if you LOAD then RUN sprites_2 instead of CHAIN

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Added VDU 23,27,16 to clear the screen and reset the sprite system
That seems to crash if called more than once after running SPRITES_5.BAS
Indicates some issues with malloc/free, possibly the bitmaps.

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Lennart Benschop has also noted that it crashes if running for more than a couple of minutes. I can replicate this.

Also:
- sprites_4.bas crashes, though it will run a lot longer before it does
- sprites_3.bas is fine; that only has 1 frame per sprite
- triangles.bas crashes in MODE 2 after 30 seconds or so

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This is looking like the increased throughput of the comms is overflowing something in FabGL.

Potential fix:

- VGAController->enableBackgroundPrimitiveExecution(true)
- VGAController->enableBackgroundPrimitiveExecution(false)

And also:

- Canvas->waitCompletion(false) after lengthy drawing operations like triangle, circle

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The above fixes have not resolved the original issue. Forked this issue breakintoprogram/agon-vdp#28