src: Drop selections that only contain whitespace on _

Fixes #2387

doc/pages/keys.asciidoc

@@ -312,7 +312,8 @@ Yanking (copying) and pasting use the *"* register by default (See <<registers#,
     option or the count parameter for tabstop
 
 *_*::
-    trim selections
+    unselect whitespace surrounding each selection, drop those that only
+    contain whitespace
 
 *<a-)>*::
     rotate selections content, if specified, the count groups selections,

src/normal.cc

@@ -1711,19 +1711,34 @@ void spaces_to_tabs(Context& context, NormalParams params)
 void trim_selections(Context& context, NormalParams)
 {
     auto& buffer = context.buffer();
-    for (auto& sel : context.selections())
+    auto& selections = context.selections();
+    Vector<int> to_remove;
+
+    for (int i = 0; i < (int)selections.size(); ++i)
     {
+        auto& sel = selections[i];
         auto beg = buffer.iterator_at(sel.min());
         auto end = buffer.iterator_at(sel.max());
         while (beg != end and is_blank(*beg))
             ++beg;
         while (beg != end and is_blank(*end))
             --end;
+
+        if (beg == end and is_blank(*beg))
+            to_remove.push_back(i);
+        else
+        {
             sel.min() = beg.coord();
             sel.max() = end.coord();
         }
     }
 
+    if (to_remove.size() == selections.size())
+        throw runtime_error{"no selections remaining"};
+    for (auto& i : to_remove | reverse())
+        selections.remove(i);
+}
+
 SelectionList read_selections_from_register(char reg, Context& context)
 {
     if (not is_basic_alpha(reg) and reg != '^')