fix(float): make screen*() functions respect floating windows

Resolves #19013.

test/functional/vimscript/screenchar_spec.lua

@@ -0,0 +1,69 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, eq, neq = helpers.clear, helpers.eq, helpers.neq
+local command, meths, funcs = helpers.command, helpers.meths, helpers.funcs
+local tbl_deep_extend = helpers.tbl_deep_extend
+
+-- Set up two overlapping floating windows
+local setup_floating_windows = function()
+  local base_opts = {
+    relative = 'editor',
+    height = 1,
+    width = 2,
+    anchor = 'NW',
+    style = 'minimal',
+    border = 'none',
+  }
+
+  local bufnr_1 = meths.create_buf(false, true)
+  meths.buf_set_lines(bufnr_1, 0, -1, true, { 'aa' })
+  local opts_1 = tbl_deep_extend('force', { row = 0, col = 0, zindex = 11 }, base_opts)
+  meths.open_win(bufnr_1, false, opts_1)
+
+  local bufnr_2 = meths.create_buf(false, true)
+  meths.buf_set_lines(bufnr_2, 0, -1, true, { 'bb' })
+  local opts_2 = tbl_deep_extend('force', { row = 0, col = 1, zindex = 10 }, base_opts)
+  meths.open_win(bufnr_2, false, opts_2)
+
+  command('redraw')
+end
+
+describe('screenchar() and family respect floating windows', function()
+  before_each(function()
+    clear()
+    -- These commands result into visible text `aabc`.
+    -- `aab` - from floating windows, `c` - from text in regular window.
+    meths.buf_set_lines(0, 0, -1, true, { 'cccc' })
+    setup_floating_windows()
+  end)
+
+  it('screenattr()', function()
+    local attr_1 = funcs.screenattr(1, 1)
+    local attr_2 = funcs.screenattr(1, 2)
+    local attr_3 = funcs.screenattr(1, 3)
+    local attr_4 = funcs.screenattr(1, 4)
+    eq(attr_1, attr_2)
+    eq(attr_1, attr_3)
+    neq(attr_1, attr_4)
+  end)
+
+  it('screenchar()', function()
+    eq(97, funcs.screenchar(1, 1))
+    eq(97, funcs.screenchar(1, 2))
+    eq(98, funcs.screenchar(1, 3))
+    eq(99, funcs.screenchar(1, 4))
+  end)
+
+  it('screenchars()', function()
+    eq({ 97 }, funcs.screenchars(1, 1))
+    eq({ 97 }, funcs.screenchars(1, 2))
+    eq({ 98 }, funcs.screenchars(1, 3))
+    eq({ 99 }, funcs.screenchars(1, 4))
+  end)
+
+  it('screenstring()', function()
+    eq('a', funcs.screenstring(1, 1))
+    eq('a', funcs.screenstring(1, 2))
+    eq('b', funcs.screenstring(1, 3))
+    eq('c', funcs.screenstring(1, 4))
+  end)
+end)