## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to #26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.

## Summary

During assistant-message streaming, blank markdown lines in the
transient active tail were prefixed with two spaces. Ratatui measured
those whitespace-only lines as two viewport rows, so list- and
table-heavy answers showed doubled vertical gaps while streaming and
then visibly compacted when finalized into scrollback.

- keep whitespace-only `StreamingAgentTailCell` lines structurally empty
while preserving nonblank message prefixes
- clear impossible hyperlink metadata when normalizing a blank tail line
- add an inline snapshot and height regression proving one blank
markdown line occupies one viewport row

Related to openai#26618, but fixes a separate live-tail row-height issue
rather than stale committed markdown content.

## How to Test

Recommended before/after reproduction:

1. Start the latest Codex build without this change.
2. Submit this exact prompt:

> Send 20 different lists: bullets vs numbered, simple vs complex with
paragraphs in between items, etc. Intertwine them with some tables and
some paragraphs.

3. While the answer streams, observe duplicated vertical gaps around
list items and paragraphs. When the answer finishes, observe the spacing
compact.
4. Start this branch with `just c` and submit the same prompt.
5. Confirm each intended blank markdown line occupies one terminal row
throughout streaming and that the spacing does not compact or jump when
the answer finishes.
6. As a focused regression, verify the sections after the first table,
especially loose lists with paragraphs between items; those blank rows
should remain stable throughout streaming.

Targeted tests:

- `just test -p codex-tui
streaming_agent_tail_blank_line_uses_one_viewport_row`
- `just test -p codex-tui history_cell::tests`

## Test Notes

- Verified the exact prompt above in a real tmux TUI using latest Codex
and this branch as the before/after comparison.
- The full `just test -p codex-tui` run completed 2,782 of 2,784 tests
successfully. Two unrelated guardian feature-flag tests fail
reproducibly in isolation because the expected `OverrideTurnContext`
message is absent.
- `just argument-comment-lint` is blocked locally by the existing Bazel
`compiler-rt` missing-header glob error; the touched Rust diff was
inspected manually for opaque positional literals.