Fix triple clicking empty line and dragging (#5068)

Fixes core issue #4957. Adds a bool to `SelectLine` allowing the
`selectLine` function to select lines that are empty. When starting a
triple selection on an empty line the initial `selectLine` returns null
because we don't see any characters, in this case we rerun `selectLine`
but short circuit with the `allow_empty_lines`. We need to run
`selectLine` with out allowing empty lines once because if there are
characters on the line we don't want to select empty space.

src/Surface.zig

@@ -3563,22 +3563,21 @@ fn dragLeftClickTriple(
     const screen = &self.io.terminal.screen;
     const click_pin = self.mouse.left_click_pin.?.*;
 
-    // Get the word under our current point. If there isn't a word, do nothing.
+    // Get the line selection under our current drag point. If there isn't a
-    const word = screen.selectLine(.{ .pin = drag_pin }) orelse return;
+    // line, do nothing.
+    const line = screen.selectLine(.{ .pin = drag_pin }) orelse return;
 
-    // Get our selection to grow it. If we don't have a selection, start it now.
+    // Get the selection under our click point. We first try to trim
-    // We may not have a selection if we started our dbl-click in an area
+    // whitespace if we've selected a word. But if no word exists then
-    // that had no data, then we dragged our mouse into an area with data.
+    // we select the blank line.
-    var sel = screen.selectLine(.{ .pin = click_pin }) orelse {
+    const sel_ = screen.selectLine(.{ .pin = click_pin }) orelse
-        try self.setSelection(word);
+        screen.selectLine(.{ .pin = click_pin, .whitespace = null });
-        return;
-    };
 
-    // Grow our selection
+    var sel = sel_ orelse return;
     if (drag_pin.before(click_pin)) {
-        sel.startPtr().* = word.start();
+        sel.startPtr().* = line.start();
     } else {
-        sel.endPtr().* = word.end();
+        sel.endPtr().* = line.end();
     }
     try self.setSelection(sel);
 }