Updates:

1. Move the debugger between the char list and the glyph list. This feels more natural when stepping through GSUB, as it prevents the debugger list from jumping around if the number of rendered glyphs change
2. Hide ineffective lookups and individual items by default. There is a little hamburger menu next to the "Shaping debugger" accordion item title that allows you to toggle this setting
3. Don't try to balance "recursing"/"recursed" messages. But also don't filter out "recursing" messages, so we can at least see to what lookup it is recursing to

(Think think this could be done now, but a little more stress-testing wouldn't hurt...)

All looks good now. I really like it.

One little thing, then clicking on collapsed steps, the glyph output before applying any of the enclosed changes is shown. This was confusing a bit at the start e.g. I expected to see the output of fina feature when clicking on it, but I got the output of ccmp feature. Maybe if the item is collapsed then the output of the last enclosed step should be shown, and when it is expanded it acts like it is now?

It would also be nice if page reload remembered which panel item was expanded or collapsed (right now it always expands Input characters and Output glyphs and collapses Shaping debugger.

I still think it would be nice to have this.

The only crucially missing bit for me is lookup names source line numbers.

Let's see how receptive the fontc team is to changes to make this possible.

googlefonts/fontc#1933

glyph output before applying any of the enclosed changes is shown. This was confusing a bit at the start

Yes, I see that is confusing, and I struggled with it a bit. The behavior you suggests sounds good but has one oddity: selecting the "folder" item would result in a different "breakpoint" when folded and unfolded. But perhaps that is still more intuitive than the current behavior.

It would also be nice if page reload remembered which panel item was expanded or collapsed (right now it always expands Input characters and Output glyphs and collapses Shaping debugger.

"Page reload" should not really be a thing for end users, so this would be mostly a convenience for us, and not for the general user. I'll think about it, but for this reason it's not high on my priority list.

glyph output before applying any of the enclosed changes is shown. This was confusing a bit at the start

Yes, I see that is confusing, and I struggled with it a bit. The behavior you suggests sounds good but has one oddity: selecting the "folder" item would result in a different "breakpoint" when folded and unfolded. But perhaps that is still more intuitive than the current behavior.

I thought about that as well, and I’m not very sure which is best, but I feel the current behavior is probably less intuitive than the other option.

It would also be nice if page reload remembered which panel item was expanded or collapsed (right now it always expands Input characters and Output glyphs and collapses Shaping debugger.

"Page reload" should not really be a thing for end users, so this would be mostly a convenience for us, and not for the general user. I'll think about it, but for this reason it's not high on my priority list.

Right.

Maybe if the item is collapsed then the output of the last enclosed step should be shown, and when it is expanded it acts like it is now?

I implemented this and I think you are right that this is much more intuitive behavior.

Maybe if the item is collapsed then the output of the last enclosed step should be shown, and when it is expanded it acts like it is now?

I implemented this and I think you are right that this is much more intuitive behavior.

It is much better now indeed.

I tested the debugger with a few fonts and I think it is pretty good now.