It took me a lot of time to make this, pls like. (YouTube Comments #1)
brainfuck, 32 bytes

,[[&lt;]&gt;[.&gt;]++++++++++.,[&gt;&gt;]&lt;[-]&lt;]


Try it online!

The same loop is used for both halves of the pattern.

Explanation:

,             Take first input character as initial line
[             Until line to output is empty:
  [&lt;]&gt;        Move to beginning of line
  [.&gt;]        Output all characters in line
  ++++++++++. Output newline
  ,           Input next character
  [&gt;&gt;]        Move two cells right if input character nonzero
  &lt;[-]        Otherwise remove last character in line
  &lt;           Move to new last character in line
]