Revert "termio: clear screen always sends form feed (0x0C)"

This reverts commit abc383854604189378bfe61239c14f263f7a14de.

See #623

src/termio/Exec.zig

@@ -379,12 +379,20 @@ pub fn clearScreen(self: *Exec, history: bool) !void {
         // Clear our scrollback
         if (history) try self.terminal.screen.clear(.history);
 
-        // Clear our screen using terminal state.
+        // If we're not at a prompt, we clear the screen manually using
+        // the terminal screen state. If we are at a prompt, we send
+        // form-feed so that the shell can repaint the entire screen.
+        if (!self.terminal.cursorIsAtPrompt()) {
+            // Clear above the cursor
             try self.terminal.screen.clear(.above_cursor);
+
+            // Exit
+            return;
+        }
     }
 
-    // We also always send form feed so that the terminal can repaint
-    // our prompt.
+    // If we reached here it means we're at a prompt, so we send a form-feed.
+    assert(self.terminal.cursorIsAtPrompt());
     try self.queueWrite(&[_]u8{0x0C});
 }