Don't abandon line before the first prompt

When a terminal in a tiling WM starts, it might start the shell before
it has reached its "final" size. So we get the terminal width,
then the terminal would be resized (to appease the tiling logic),
and then we would print the abandon line with the omitted newline
char, only if the size got smaller (likely!), we would overflow the
line and land on the next.

So what we do is a bit of a hack: We don't abandon the first line.

This means that `printf %s foo; fish` will overwrite the `foo`, but
that's a super small problem and I don't see another way around this.

Fixes #7893.

src/reader.cpp

@@ -586,6 +586,9 @@ class reader_data_t : public std::enable_shared_from_this<reader_data_t> {
     /// HACK: A flag to reset the loop state from the outside.
     bool reset_loop_state{false};
 
+    /// Whether this is the first prompt.
+    bool first_prompt{true};
+
     /// The representation of the current screen contents.
     screen_t screen;
 
@@ -3881,7 +3884,20 @@ maybe_t<wcstring> reader_data_t::readline(int nchars_or_0) {
         }
     }
 
-    s_reset_abandoning_line(&screen, termsize_last().width);
+    // HACK: Don't abandon line for the first prompt, because
+    // if we're started with the terminal it might not have settled,
+    // so the width is quite likely to be in flight.
+    //
+    // This means that `printf %s foo; fish` will overwrite the `foo`,
+    // but that's a smaller problem than having the omitted newline char
+    // appear constantly.
+    //
+    // I can't see a good way around this.
+    if (!first_prompt) {
+        s_reset_abandoning_line(&screen, termsize_last().width);
+    }
+    first_prompt = false;
+
     event_fire_generic(parser(), L"fish_prompt");
     exec_prompt();