From 1e693f06a6556b87ed4f56a3635a0c10640b1f92 Mon Sep 17 00:00:00 2001
From: mitchell <70453897+orbitalquark@users.noreply.github.com>
Date: Sun, 11 Apr 2021 22:51:59 -0400
Subject: Save/restore view state when undoing/redoing full-buffer changes. For
 example external code formatting commands that replace buffer contents.

---
 modules/textadept/history.lua | 21 ++++++++++++++++++---
 test/test.lua                 | 27 +++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/modules/textadept/history.lua b/modules/textadept/history.lua
index da9f5885..05081ddd 100644
--- a/modules/textadept/history.lua
+++ b/modules/textadept/history.lua
@@ -33,20 +33,35 @@ local view_history = setmetatable({}, {
   end
 })
 
+local restore_position, first_visible_line = false, nil
+-- Restore position after a full-buffer undo/redo operation, e.g. after replacing buffer contents
+-- with a formatting command and then performing an undo.
+events.connect(events.UPDATE_UI, function(updated)
+  if not restore_position or updated & buffer.UPDATE_SELECTION == 0 then return end
+  restore_position = false
+  M.back()
+  view.first_visible_line, first_visible_line = first_visible_line, nil
+end)
+
 -- Listens for text insertion and deletion events and records their locations.
 events.connect(events.MODIFIED, function(position, mod, text, length)
   local buffer = buffer
   -- Only interested in text insertion or deletion.
   if mod & buffer.MOD_INSERTTEXT > 0 then
-    if length == buffer.length then return end -- ignore file loading
+    if length == buffer.length then
+      if mod & buffer.MULTILINEUNDOREDO > 0 then restore_position = true end
+      return -- ignore file loading or replacing buffer contents
+    end
     position = position + length
   elseif mod & buffer.MOD_DELETETEXT > 0 then
     if buffer.length == 0 then return end -- ignore replacing buffer contents
+  elseif mod & (buffer.PERFORMED_UNDO | buffer.PERFORMED_REDO) > 0 and
+      (mod & buffer.MOD_BEFOREDELETE > 0) and length == buffer.length then
+    first_visible_line = view.first_visible_line -- save for potential undo before it's lost
   else
     return
   end
-  -- Ignore undo/redo.
-  if mod & (buffer.PERFORMED_UNDO | buffer.PERFORMED_REDO) > 0 then return end
+  if mod & (buffer.PERFORMED_UNDO | buffer.PERFORMED_REDO) > 0 then return end -- ignore undo/redo
   M.record(nil, buffer:line_from_position(position), buffer.column[position])
 end)
 
diff --git a/test/test.lua b/test/test.lua
index e9c6bf75..4e6b5441 100644
--- a/test/test.lua
+++ b/test/test.lua
@@ -2932,6 +2932,33 @@ function test_history_print_buffer()
   ui.tabs = tabs -- restore
 end
 
+function test_history_undo_full_buffer_change()
+  buffer.new()
+  local lines = {}
+  for i = 99, 1, -1 do lines[#lines + 1] = tostring(i) end
+  buffer:add_text(table.concat(lines, '\n'))
+  buffer:goto_line(50)
+  buffer:add_text('1')
+  textadept.editing.filter_through('sort -n')
+  ui.update()
+  assert(buffer:get_line(buffer:line_from_position(buffer.current_pos)) ~= '150\n', 'not sorted')
+  local first_visible_line = view.first_visible_line
+  buffer:undo()
+  -- Verify the view state was restored.
+  ui.update()
+  if CURSES then events.emit(events.UPDATE_UI, buffer.UPDATE_SELECTION) end
+  assert_equal(buffer:line_from_position(buffer.current_pos), 50)
+  assert_equal(buffer:get_line(buffer:line_from_position(buffer.current_pos)), '150\n')
+  assert_equal(view.first_visible_line, first_visible_line)
+  buffer:redo()
+  -- Verify the previous view state was kept.
+  ui.update()
+  if CURSES then events.emit(events.UPDATE_UI, buffer.UPDATE_SELECTION) end
+  assert_equal(buffer:line_from_position(buffer.current_pos), 50)
+  assert_equal(view.first_visible_line, first_visible_line)
+  buffer:close(true)
+end
+
 function test_macro_record_play_save_load()
   textadept.macros.save() -- should not do anything
   textadept.macros.play() -- should not do anything
-- 
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