Reported from https://groups.google.com/d/msgid/bluestatic/84a11ca5-eea3-4d6d-bfdc-c52068b1cf42%40googlegroups.com

"""
I'm unable to add breakpoints in large PHP files. For example, I tried to add a breakpoint at line 843 in a file of the Drupal Core.

Reproduce:
- Clone/Install Drupal 8 Core (https://git.drupalcode.org/project/drupal, 8.8.X);
- Go to breakpoint Tab;
- Click the `+` symbol to add a "Line Number" (breakpoint);
- Select the file in the file browser;
- Click ok;
Nothing will happen; the file doesn't appear in the list of breakpoints.
"""

What is happening is the standard out pipe buffer is getting full because MacGDBp is not reading from it as the php command is producing output.

Compare version 1.5, which would start reading from the pipe buffer immediately after launching: https://src.bluestatic.org/?p=macgdbp.git;a=blob;f=Source/BSSourceView.mm;h=4042c5d361dc64b79339da9decbd8572b0a95486;hb=0508dff4d4670e79a64e424e4a66af1d2003468e#l82

Versus version 2.0, which waits until the task terminates to read from the pipe buffer: https://src.bluestatic.org/?p=macgdbp.git;a=blob;f=Source/BSSourceView.mm;h=7899dfc1ad341dcea8461880dbfa95841c011675;hb=8eef7c99bed2f60c4daf9b9afc370b0ef413bdc2#l222

In 2.0, you can see the php process running from MacGDBp:

% ps ax | grep php
54384 ?? S 0:00.06 /usr/bin/php --syntax-highlight --define highlight.string="#fb2025" --define highlight.comment="#fc8108" --define highlight.default="#0a5ffe" --define highlight.html="#7b7b80" /Users/rsesek/Downloads/core_lib_Drupal_Core_Entity_Sql_SqlContentEntityStorageSchema.php

And sampling it shows waiting on the write() system call:
9002 Thread_23592737 DispatchQueue_1: com.apple.main-thread (serial)
9002 start (in libdyld.dylib) + 1 [0x7fff70d4c3d5]
9002 ??? (in php) load address 0x100000000 + 0x40a268 [0x10040a268]
9002 ??? (in php) load address 0x100000000 + 0x40aecf [0x10040aecf]
9002 zend_highlight (in php) + 117 [0x10036564e]
9002 lex_scan (in php) + 705 [0x10033d7af]
9002 zend_error (in php) + 124 [0x1003710f2]
9002 ??? (in php) load address 0x100000000 + 0x371386 [0x100371386]
9002 ??? (in php) load address 0x100000000 + 0x21a65b [0x10021a65b]
9002 ??? (in php) load address 0x100000000 + 0x31b022 [0x10031b022]
9002 php_printf (in php) + 163 [0x1003189db]
9002 php_output_write (in php) + 67 [0x10032aa8a]
9002 ??? (in php) load address 0x100000000 + 0x32ab82 [0x10032ab82]
9002 ??? (in php) load address 0x100000000 + 0x40b646 [0x10040b646]
9002 sapi_cli_single_write (in php) + 78 [0x100409d03]
9002 write (in libsystem_kernel.dylib) + 10 [0x7fff70e84f46]

MacGDBp needs to start reading from the pipe after launching the task, not after waiting for it to complete.