fix: screenchar()/screenstring() with multigrid #32494

Problem:
- When multigrid is enabled, screenchar()/screenstring() functions return wrong
  results. See https://github.com/neovide/neovide/issues/2569
- `screenstring()` executed via RPC in child Nvim process, doesn't recognize
  floating windows.

Solution:
In ui_comp_get_grid_at_coord(), also iterate window grids.

src/nvim/ui_compositor.c

@@ -333,6 +333,14 @@ ScreenGrid *ui_comp_get_grid_at_coord(int row, int col)
       return grid;
     }
   }
+
+  FOR_ALL_WINDOWS_IN_TAB(wp, curtab) {
+    ScreenGrid *grid = &wp->w_grid_alloc;
+    if (row >= grid->comp_row && row < grid->comp_row + grid->rows
+        && col >= grid->comp_col && col < grid->comp_col + grid->cols) {
+      return grid;
+    }
+  }
   return &default_grid;
 }
 

test/functional/vimscript/screenchar_spec.lua

@@ -1,5 +1,6 @@
 local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
+local Screen = require('test.functional.ui.screen')
 
 local clear, eq, neq = n.clear, t.eq, t.neq
 local command, api, fn = n.command, n.api, n.fn
@@ -30,8 +31,10 @@ local setup_floating_windows = function()
 end
 
 describe('screenchar() and family respect floating windows', function()
+  local function with_ext_multigrid(multigrid)
     before_each(function()
       clear()
+      Screen.new(40, 7, { ext_multigrid = multigrid })
       -- These commands result into visible text `aabc`.
       -- `aab` - from floating windows, `c` - from text in regular window.
       api.nvim_buf_set_lines(0, 0, -1, true, { 'cccc' })
@@ -68,4 +71,12 @@ describe('screenchar() and family respect floating windows', function()
       eq('b', fn.screenstring(1, 3))
       eq('c', fn.screenstring(1, 4))
     end)
+  end
+
+  describe('with ext_multigrid', function()
+    with_ext_multigrid(true)
+  end)
+  describe('without ext_multigrid', function()
+    with_ext_multigrid(false)
+  end)
 end)