font_size_pt state bleeds across TextRegion boundaries when using ln()

[Pawansingh3889]

Bug Description

When using TextRegion with ln(h), the internal font_size_pt state from a previous paragraph can affect line spacing in the next paragraph. This is the root cause behind #1786 (which I submitted a fix for in PR #1803).

Root Cause Analysis

The issue is deeper than the ln() fix — TextRegion._render_paragraph() modifies self.pdf.font_size_pt during rendering, but does not restore it when the paragraph ends. If the next operation reads font_size_pt (e.g., ln() without an explicit h), it gets the value from the last line of the previous paragraph rather than the current font setting.

Minimal Reproduction

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()

with pdf.text_region() as region:
    pdf.set_font("Helvetica", size=24)
    region.paragraph().write("Large heading")
    region.ln()  # uses font_size_pt from the 24pt paragraph

    pdf.set_font("Helvetica", size=10)
    region.paragraph().write("Small body text")
    region.ln()  # should use 10pt spacing, but may inherit 24pt

Expected Behavior

Each ln() call should use the current font size at the time of the call, not the font size from the last rendered paragraph.

Environment

• fpdf2 2.8.3
• Python 3.11

[andersonhc]

Hi @Pawansingh3889, thanks for the report.

In my testing, I could reproduce a related issue, but only in a narrower scenario than described.

If text_columns() is the first text rendered on the page, rendering the region can leak font state back onto the FPDF instance. In that case, pdf.font_size_pt is left modified after the region finishes rendering.

I could not reproduce the same leak once other text had already been rendered on the page beforehand.

This is the minimal example I found:

from fpdf import FPDF

pdf = FPDF()
pdf.add_page()
pdf.set_font("Helvetica", size=12)

cols = pdf.text_columns()
with cols:
    pdf.set_font("Helvetica", size=24)
    with cols.paragraph() as p:
        p.write("Large heading")

    pdf.set_font("Helvetica", size=10)
    with cols.paragraph() as p:
        p.write("Small body text")

pdf.ln()
pdf.cell(text=f"after columns size is {pdf.font_size_pt}")

pdf.output("issue1804.pdf")

With this example, the output is after columns size is 24 instead of the expected 12.

If the problem you are describing is different, a minimal reproducible example for that specific case would be helpful. Contributions toward a fix are welcome as well.