fix(event-loop): call vpeekc() directly first to check for character

Expand mappings first by calling `vpeekc()` directly.

src/nvim/eval/funcs.c

@@ -3180,7 +3180,7 @@ static void getchar_common(typval_T *argvars, typval_T *rettv)
     if (argvars[0].v_type == VAR_UNKNOWN) {
       // getchar(): blocking wait.
       // TODO(bfredl): deduplicate shared logic with state_enter ?
-      if (!(char_avail() || using_script() || input_available())) {
+      if (!char_avail()) {
         (void)os_inchar(NULL, 0, -1, 0, main_loop.events);
         if (!multiqueue_empty(main_loop.events)) {
           state_handle_k_event();

src/nvim/state.c

@@ -39,10 +39,16 @@ void state_enter(VimState *s)
     int key;
 
 getkey:
-    if (char_avail() || using_script() || input_available()) {
-      // Don't block for events if there's a character already available for
-      // processing. Characters can come from mappings, scripts and other
-      // sources, so this scenario is very common.
+    // Expand mappings first by calling vpeekc() directly.
+    // - If vpeekc() returns non-NUL, there is a character already available for processing, so
+    //   don't block for events. vgetc() may still block, in case of an incomplete UTF-8 sequence.
+    // - If vpeekc() returns NUL, vgetc() will block, and there are three cases:
+    //   - There is no input available.
+    //   - All of available input maps to an empty string.
+    //   - There is an incomplete mapping.
+    //   A blocking wait for a character should only be done in the third case, which is the only
+    //   case of the three where typebuf.tb_len > 0 after vpeekc() returns NUL.
+    if (vpeekc() != NUL || typebuf.tb_len > 0) {
       key = safe_vgetc();
     } else if (!multiqueue_empty(main_loop.events)) {
       // Event was made available after the last multiqueue_process_events call
@@ -55,9 +61,11 @@ getkey:
       // mapping engine.
       (void)os_inchar(NULL, 0, -1, 0, main_loop.events);
       // If an event was put into the queue, we send K_EVENT directly.
-      key = !multiqueue_empty(main_loop.events)
-            ? K_EVENT
-            : safe_vgetc();
+      if (!multiqueue_empty(main_loop.events)) {
+        key = K_EVENT;
+      } else {
+        goto getkey;
+      }
     }
 
     if (key == K_EVENT) {

test/functional/ui/input_spec.lua

@@ -140,6 +140,25 @@ describe('input utf sequences that contain CSI (0x9B)', function()
   end)
 end)
 
+describe('input split utf sequences', function()
+  it('ok', function()
+    local str = '►'
+    feed('i' .. str:sub(1, 1))
+    helpers.sleep(10)
+    feed(str:sub(2, 3))
+    expect('►')
+  end)
+
+  it('can be mapped', function()
+    command('inoremap ► E296BA')
+    local str = '►'
+    feed('i' .. str:sub(1, 1))
+    helpers.sleep(10)
+    feed(str:sub(2, 3))
+    expect('E296BA')
+  end)
+end)
+
 describe('input non-printable chars', function()
   after_each(function()
     os.remove('Xtest-overwrite')

test/functional/vimscript/timer_spec.lua

@@ -272,4 +272,12 @@ describe('timers', function()
     ]]
     eq("Vim(call):E48: Not allowed in sandbox", exc_exec("sandbox call timer_start(0, 'Scary')"))
   end)
+
+  it('can be triggered after an empty string <expr> mapping', function()
+    local screen = Screen.new(40, 6)
+    screen:attach()
+    command([=[imap <expr> <F2> [timer_start(0, { _ -> execute("throw 'x'", "") }), ''][-1]]=])
+    feed('i<F2>')
+    screen:expect({any='E605: Exception not caught: x'})
+  end)
 end)