ENB: A greatly simplified colorizer!

[Edward K. Ream]

PR #4147 fixes a medium-scale performance bug. While doing so, I discovered two ways of collapsing the complexity of the colorizer.

Naive expectations

We have the right to expect that the qsh will work as follows:

- When changing nodes, the qsh should call _redraw for every line of p.b.

- Otherwise, we expect the qsh to issue highly optimized calls to _redraw when users edit text.

In fact, the qsh called _redraw for all lines of p.b whenever the user changed p.b. Ouch!

The PR identifies two culprits as the cause of the performance bug.

jedit.colorizer

Previously, this method always forced a full redraw. That makes no sense! Now it contains the following new code:

# #4146: Fully recolor p.b *only* if necessary.

old_language = self.language

self.updateSyntaxColorer(p)

if p.v == self.old_v and self.language == old_language:

    return

jedit._redraw

Imagine my surprise when I found the following code at the start of jedit._redraw:

block_n = self.currentBlockNumber()

n = self.prevState()

if p.v == self.old_v:

    new_language = self.n2languageDict.get(n)

    if new_language != self.language:

        self.language = new_language

        self.init()

else:

    self.updateSyntaxColorer(p) # Force a full recolor

    assert self.language

    self.init_all_state(p.v)

    self.init()

Huh??? My intense study of the code showed me that jedit._redraw was a callback for qsh.highlightBlock. Nothing else should ever be calling jedit._redraw. That's why I added the leading underscore.

Forcing the qsh to do a full redraw inside _redraw is utterly misguided. It is a gross interference with the qsh!

I immediately removed this bizarre code. All of a sudden, for the first time in Leo's history, the qsh worked as one would naively expect!

Summary

How did Leo's legacy syntax colorer have any chance of working? It's a mystery.

I fixed the two culprits as follows:

- jedit.colorize forces a full redraw only when Leo changes nodes.

- jedit._redraw never forces a full redraw.

After these changes, the qsh operates exactly as one would naively expect.

Edward