fix(terminal): reset w_leftcol after resizing terminal

Problem: windows may scroll horizontally upon resize using the old terminal
size, which may be unnecessary and cause the content to be partially out-of-view.

Solution: reset the horizontal scroll after resizing.

src/nvim/terminal.c

@@ -2210,13 +2210,24 @@ static void refresh_terminal(Terminal *term)
   }
   linenr_T ml_before = buf->b_ml.ml_line_count;
 
-  refresh_size(term, buf);
+  bool resized = refresh_size(term, buf);
   refresh_scrollback(term, buf);
   refresh_screen(term, buf);
 
   int ml_added = buf->b_ml.ml_line_count - ml_before;
   adjust_topline_cursor(term, buf, ml_added);
 
+  // Resized window may have scrolled horizontally to keep its cursor in-view using the old terminal
+  // size. Reset the scroll, and let curs_columns correct it if that sends the cursor out-of-view.
+  if (resized) {
+    FOR_ALL_TAB_WINDOWS(tp, wp) {
+      if (wp->w_buffer == buf && wp->w_leftcol != 0) {
+        wp->w_leftcol = 0;
+        curs_columns(wp, true);
+      }
+    }
+  }
+
   // Copy pending events back to the main event queue
   multiqueue_move_events(main_loop.events, term->pending.events);
 }
@@ -2287,10 +2298,10 @@ static void refresh_timer_cb(TimeWatcher *watcher, void *data)
   unblock_autocmds();
 }
 
-static void refresh_size(Terminal *term, buf_T *buf)
+static bool refresh_size(Terminal *term, buf_T *buf)
 {
   if (!term->pending.resize || term->closed) {
-    return;
+    return false;
   }
 
   term->pending.resize = false;
@@ -2299,6 +2310,7 @@ static void refresh_size(Terminal *term, buf_T *buf)
   term->invalid_start = 0;
   term->invalid_end = height;
   term->opts.resize_cb((uint16_t)width, (uint16_t)height, term->opts.data);
+  return true;
 }
 
 void on_scrollback_option_changed(Terminal *term)

test/functional/terminal/window_spec.lua

@@ -14,6 +14,7 @@ local eq = t.eq
 local eval = n.eval
 local skip = t.skip
 local is_os = t.is_os
+local testprg = n.testprg
 
 describe(':terminal window', function()
   before_each(clear)
@@ -33,6 +34,46 @@ describe(':terminal window', function()
     eq({ 1, 1, 1 }, eval('[&l:wrap, &wrap, &g:wrap]'))
     eq({ 1, 1, 1 }, eval('[&l:list, &list, &g:list]'))
   end)
+
+  it('resets horizontal scroll on resize #35331', function()
+    local screen = tt.setup_screen(0, { testprg('shell-test'), 'INTERACT' })
+    command('set statusline=%{win_getid()} splitright')
+    screen:expect([[
+      interact $ ^                                       |
+                                                        |*5
+      {5:-- TERMINAL --}                                    |
+    ]])
+    feed_data(('A'):rep(30))
+    screen:expect([[
+      interact $ AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA^         |
+                                                        |*5
+      {5:-- TERMINAL --}                                    |
+    ]])
+    command('vnew | wincmd p')
+    screen:expect([[
+      interact $ AAAAAAAAAAAAA│                         |
+      AAAAAAAAAAAAAAAAA^       │{100:~                        }|
+                              │{100:~                        }|*3
+      {120:1000                     }{2:1001                     }|
+      {5:-- TERMINAL --}                                    |
+    ]])
+
+    feed([[<C-\><C-N><C-W>o]])
+    screen:expect([[
+      interact $ AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA         |
+      ^                                                  |
+                                                        |*5
+    ]])
+    -- Window with less room scrolls anyway to keep its cursor in-view.
+    feed('gg$20<C-W>v')
+    screen:expect([[
+      interact $ AAAAAAAAAAAAAAAAAA│$ AAAAAAAAAAAAAAAAA^A|
+      AAAAAAAAAAAA                 │AAA                 |
+                                   │                    |*3
+      {119:1000                          }{120:1002                }|
+                                                        |
+    ]])
+  end)
 end)
 
 describe(':terminal window', function()