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fix: carry forward highlighted hunks on reparse to reduce flicker (#138)

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## Problem

Toggling large diffs via fugitive's `=` caused the top of the buffer to
re-render and glitch. `ensure_cache` always created a new cache entry
with
`pending_clear=true` and `highlighted={}`, forcing `on_win` to clear and
re-highlight every visible hunk — including stable ones above the toggle
point that never changed.

## Solution

On reparse, compare old and new hunk lists using a prefix + suffix
matching
strategy. Hunks that match (same filename, line count, and sampled
content)
carry forward their `highlighted` state so `on_win` skips them.
Comparison
is O(1) per hunk. Only runs when the old entry had
`pending_clear=false`;
`invalidate_cache`/`ColorScheme` paths still force full re-highlight.

Closes #131
This commit is contained in:
Barrett Ruth 2026-02-25 12:44:57 -05:00 • committed by GitHub
parent d45ffd279b
commit 6040c054cb
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2 changed files with 201 additions and 2 deletions
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128
spec/decoration_provider_spec.lua
View file

@ -139,6 +139,134 @@ describe('decoration_provider', function()
end)
end)
describe('hunk stability', function()
it('carries forward highlighted for stable hunks on section expansion', function()
local bufnr = create_buffer({
'M test.lua',
'@@ -1,2 +1,2 @@',
' local x = 1',
'-local y = 2',
'+local y = 3',
'@@ -10,2 +10,3 @@',
' function M.foo()',
'+ return true',
' end',
})
diffs.attach(bufnr)
local entry = diffs._test.hunk_cache[bufnr]
assert.are.equal(2, #entry.hunks)
entry.pending_clear = false
entry.highlighted = { [1] = true, [2] = true }
vim.api.nvim_buf_set_lines(bufnr, 5, 5, false, {
'@@ -5,1 +5,2 @@',
' local z = 4',
'+local w = 5',
})
diffs._test.ensure_cache(bufnr)
local updated = diffs._test.hunk_cache[bufnr]
assert.are.equal(3, #updated.hunks)
assert.is_true(updated.highlighted[1] == true)
assert.is_nil(updated.highlighted[2])
assert.is_true(updated.highlighted[3] == true)
assert.is_false(updated.pending_clear)
delete_buffer(bufnr)
end)
it('carries forward highlighted for stable hunks on section collapse', function()
local bufnr = create_buffer({
'M test.lua',
'@@ -1,2 +1,2 @@',
' local x = 1',
'-local y = 2',
'+local y = 3',
'@@ -5,1 +5,2 @@',
' local z = 4',
'+local w = 5',
'@@ -10,2 +10,3 @@',
' function M.foo()',
'+ return true',
' end',
})
diffs.attach(bufnr)
local entry = diffs._test.hunk_cache[bufnr]
assert.are.equal(3, #entry.hunks)
entry.pending_clear = false
entry.highlighted = { [1] = true, [2] = true, [3] = true }
vim.api.nvim_buf_set_lines(bufnr, 5, 8, false, {})
diffs._test.ensure_cache(bufnr)
local updated = diffs._test.hunk_cache[bufnr]
assert.are.equal(2, #updated.hunks)
assert.is_true(updated.highlighted[1] == true)
assert.is_true(updated.highlighted[2] == true)
assert.is_false(updated.pending_clear)
delete_buffer(bufnr)
end)
it('bypasses carry-forward when pending_clear was true', function()
local bufnr = create_buffer({
'M test.lua',
'@@ -1,2 +1,2 @@',
' local x = 1',
'-local y = 2',
'+local y = 3',
'@@ -10,2 +10,3 @@',
' function M.foo()',
'+ return true',
' end',
})
diffs.attach(bufnr)
local entry = diffs._test.hunk_cache[bufnr]
entry.highlighted = { [1] = true, [2] = true }
entry.pending_clear = true
vim.api.nvim_buf_set_lines(bufnr, 5, 5, false, {
'@@ -5,1 +5,2 @@',
' local z = 4',
'+local w = 5',
})
diffs._test.ensure_cache(bufnr)
local updated = diffs._test.hunk_cache[bufnr]
assert.are.same({}, updated.highlighted)
assert.is_true(updated.pending_clear)
delete_buffer(bufnr)
end)
it('does not carry forward when all hunks changed', function()
local bufnr = create_buffer({
'M test.lua',
'@@ -1,2 +1,2 @@',
' local x = 1',
'-local y = 2',
'+local y = 3',
})
diffs.attach(bufnr)
local entry = diffs._test.hunk_cache[bufnr]
entry.pending_clear = false
entry.highlighted = { [1] = true }
vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, {
'M other.lua',
'@@ -1,1 +1,2 @@',
' local a = 1',
'+local b = 2',
})
diffs._test.ensure_cache(bufnr)
local updated = diffs._test.hunk_cache[bufnr]
assert.is_nil(updated.highlighted[1])
assert.is_false(updated.pending_clear)
delete_buffer(bufnr)
end)
end)
describe('multiple hunks in cache', function()
it('stores all parsed hunks for a multi-hunk buffer', function()
local bufnr = create_buffer({