Problem:
While the fold level computation is incremental, the evaluation of the
foldexpr is done on the full buffer. Despite that the foldexpr reads
from the cache, it can take tens of milliseconds for moderately big (10K
lines) buffers.
Solution:
Track the range of lines on which the foldexpr should be evaluated.
* Collect on_bytes and flush at the invocation of the scheduled callback
to take account of commands that triggers multiple on_bytes.
* More accurately track movement of folds so that foldexpr returns
reasonable values even when the scheduled computation is not run yet.
* Start computing folds from the line above (+ foldminlines) the changed
lines to handle the folds that are removed due to the size limit.
* Shrink folds that end at the line at which another fold starts to
assign proper level to that line.
* Use level '=' for lines that are not computed yet.
Problem: Visual highlight is inconsistent on a folded line with
treesitter foldtext.
Solution: Don't added Folded highlight as it is already in background.
Problem:
Folds are opened when the visible range changes even if there are no
modifications to the buffer, e.g, when using zM for the first time. If
the parsed tree was invalid, on_win re-parses and gets empty tree
changes, which triggers fold updates.
Solution:
Don't update folds in on_changedtree if there are no changes.
Problem:
With treesitter fold, InsertLeave can be slow, because a single session
of insert mode may schedule multiple fold updates in on_bytes and
on_changedtree.
Solution:
Don't create duplicate autocmds.
Problem:
Treesitter highlighting is slow for large files with lots of injections.
Solution:
Only parse injections we are going to render during a redraw cycle.
---
- `LanguageTree:parse()` will no longer parse injections by default and
now requires an explicit range argument to be passed.
- `TSHighlighter` now parses injections incrementally during on_win
callbacks for the line range being rendered.
- Plugins which require certain injections to be parsed must run
`parser:parse({ start_row, end_row })` before using the tree.
Problem: When using treesitter foldexpr,
* :diffput/get open diff folds, and
* folds are not updated in other windows that contain the updated
buffer.
Solution: Update folds in all windows that contain the updated buffer
and use expr foldmethod.
Problem: Treesitter fold is not updated if treesitter hightlight is not
active. More precisely, updating folds requires `LanguageTree:parse()`.
Solution: Call `parse()` before computing folds and compute folds when
lines are added/removed.
This doesn't guarantee correctness of the folds, because some changes
that don't add/remove line won't update the folds even if they should
(e.g. adding pair of braces). But it is good enough for most cases,
while not introducing big overhead.
Also, if highlighting is active, it is likely that
`TSHighlighter._on_buf` already ran `parse()` (or vice versa).
