Debug screen is black "in front of" the beam

[mattgodbolt]

Hitting a breakpoint or ctrl-HOME now yields a black screen post-the virtual CRTC beam. To repro:

• MODE 2
• ctrl-s 0 4 0 0 0
• hit ctrl-home
• notice the screen post the beam location is black.

I think this is probably a result of the improvements to the CRTC emulation, but it's convenient to see the screen when debugging :)

[mattgodbolt]

@richtw1 quite likes this: maybe it should be configurable? or the "remaining" screen should be like 30% faded out or something?

[richtw1]

I don't think I'd bother keeping it TBH. If it's more useful to see the entire screen then that's what it should do. Sometimes a million configurable options just needlessly complicates!

[richtw1]

That said, this isn't obviously easy to fix, because it's an inevitable consequence of clearing the texture at the start, instead of blanking scanlines as they're rasterised. This gave a reasonable performance gain, so it would be a shame to put it back.

Other possibilities:

• always pause when raster at the bottom of the screen (might lead to stopping in the same point in execution each time?)
• separate pause / screenshot button (which doesn't enter the debugger, just waits until the bottom of the screen and then stops until you press Go again)

[mattgodbolt]

Agreed it's not an easy fix. Just taking these comments:

1. Not really an option as "pause" is the same as breakpoint, and when debugging things it's also useful to know what's on the screen. In particular I use this a lot when I do presentations on jsbeeb (e.g. https://mattgodbolt.github.io/bbc-micro-emulation/avast/#/18 which I run on purpose to crash on the "undefined" opcode).
2. This is cool for screenshots so 👍 but less useful in general for debugging.

Keeping the "old" screen around in a debug mode might be an option. I already have some (expensive) debugging things knocking around that are disabled by default, and so "keep old screen around when debugging" might be an option. So long as I can put ?debug or something in the URL I'll be happy :)

[mattgodbolt]

(that said the "old" screen around could be used to e.g. do phosphor simulation)