fix: cancel update if view becomes out of focus

[8veStory]

Fixes #462.

I work in a large monorepo and noticed that when :DiffviewOpen was used, and you switched to another tab, you would switch back to the Diffview window.

Reason for this being that inside the async function DiffView:update_file(), it is possible for the line local err, new_files = await(self:get_updated_files()) to take a long amount of time in large git repos. This meant it's possible for a user to switch to another tab prior to the async function completing, in which at the end of the function the user's window is switched to the DiffView.

I'm not entirely familiar with Lua but I made a very rough PR that changes the order of when re-rendering the panels work to abate this issue.

[sindrets]

Thanks for the PR!

The plugin probably needs a bigger refactor to better deal with synchronization issues. But in this case we should be able to improve the behavior considerably with a smaller change I think.

The problem isn't with rendering. Panels can render without changing focus. I think we should just cancel the file update if the view is no longer in focus when get_updated_files() settles. We can determine that by looking at the current tab page before and after awaiting this method. How about something like this:

diff --git a/lua/diffview/scene/views/diff/diff_view.lua b/lua/diffview/scene/views/diff/diff_view.lua
index 283fed9..84d9941 100644
--- a/lua/diffview/scene/views/diff/diff_view.lua
+++ b/lua/diffview/scene/views/diff/diff_view.lua
@@ -327,6 +327,12 @@ DiffView.update_files = debounce.debounce_trailing(
   async.wrap(function(self, callback)
     await(async.scheduler())
 
+    -- Never update unless the view is in focus
+    if self.tabpage ~= api.nvim_get_current_tabpage() then
+      callback({ "The update was cancelled." })
+      return
+    end
+
     ---@type PerfTimer
     local perf = PerfTimer("[DiffView] Status Update")
     self:ensure_layout()
@@ -344,19 +350,24 @@ DiffView.update_files = debounce.debounce_trailing(
     end
 
     local index_stat = pl:stat(pl:join(self.adapter.ctx.dir, "index"))
-    local last_winid = api.nvim_get_current_win()
 
     ---@type string[]?, FileDict
     local err, new_files = await(self:get_updated_files())
+    await(async.scheduler())
 
     if err then
       utils.err("Failed to update files in a diff view!", true)
       logger:error("[DiffView] Failed to update files!")
-      await(async.scheduler())
       callback(err)
       return
     end
 
+    -- Stop the update if the view is no longer in focus.
+    if self.tabpage ~= api.nvim_get_current_tabpage() then
+      callback({ "The update was cancelled." })
+      return
+    end
+
     perf:lap("received new file list")
 
     local files = {
@@ -365,8 +376,6 @@ DiffView.update_files = debounce.debounce_trailing(
       { cur_files = self.files.staged, new_files = new_files.staged },
     }
 
-    await(async.scheduler())
-
     for _, v in ipairs(files) do
       -- We diff the old file list against the new file list in order to find
       -- the most efficient way to morph the current list into the new. This
@@ -473,10 +482,6 @@ DiffView.update_files = debounce.debounce_trailing(
     end
     self:set_file(self.panel.cur_file or self.panel:next_file(), false, not self.initialized)
 
-    if api.nvim_win_is_valid(last_winid) then
-      api.nvim_set_current_win(last_winid)
-    end
-
     self.update_needed = false
     perf:time()
     logger:lvl(5):debug(perf)

[sindrets]

@8veStory sorry for hijacking your PR. I think this should solve the issue you described, but let me know if you still have any problems, and we can iterate on this.