fix zero height ln() after empty cell(), py-pdf#601

CHANGELOG.md

@@ -31,6 +31,7 @@ This can also be enabled programmatically with `warnings.simplefilter('default',
 ### Removed
 - `open()` & `close()` methods, that were only used internally and should never have been called by end-user code
 ### Fixed
+- After an "empty" cell(), ln() applied a line height of zero [#601](https://github.com/PyFPDF/fpdf2/issues/601)
 - when using `multi_cell()` with `max_line_height` to render multiline text, the last line is now rendered like all the others
 - templates don't leak graphics state changes to their surroundings anymore; [#570](https://github.com/PyFPDF/fpdf2/issues/570)
 - automatic page break is never performed on an empty page (when the Y position is at the top margin)

fpdf/fpdf.py

@@ -2935,7 +2935,9 @@ def _render_styled_text_line(
                 s = " ".join(sl)
             # pylint: enable=too-many-boolean-expressions
             self._out(s)
-        self._lasth = h
+        # If the text is empty, h = max_font_size ends up as 0.
+        # We still need a valid default height for self.ln() (issue #601).
+        self._lasth = h or self.font_size
 
         # XPos.LEFT -> self.x stays the same
         if new_x == XPos.RIGHT:

test/text/test_cell.py

@@ -260,3 +260,26 @@ def test_cell_curfont_leak(tmp_path):  # issue #475
     pdf.cell(txt="XYZ012abc,-", new_x="LEFT", new_y="NEXT")
     pdf.cell(txt="3,7E-05", new_x="LEFT", new_y="NEXT")
     assert_pdf_equal(pdf, HERE / "cell_curfont_leak.pdf", tmp_path)
+
+
+# pylint: disable=protected-access
+def test_cell_lasth(tmp_path):  # issue #601
+    pdf = FPDF()
+    pdf.add_page()
+    pdf.set_font("helvetica", size=18)
+    pdf.set_fill_color(255, 255, 0)
+    pdf.cell(w=100, txt="Hello world", fill=True)
+    print(pdf._lasth)  # prints: 6.35
+    pdf.ln()
+    assert pdf._lasth == 6.35, f"pdf._lasth ({pdf._lasth}) != 5.35"
+    pdf.set_fill_color(255, 0, 255)
+    pdf.cell(w=100, txt="Hello world", h=50, fill=True)
+    pdf.ln()
+    assert pdf._lasth == 50, f"pdf._lasth ({pdf._lasth}) != 50 after cell(h=50)"
+    pdf.set_fill_color(0, 255, 255)
+    pdf.cell(w=100, txt="Hello world", fill=True)
+    pdf.cell(w=100, txt="")
+    pdf.ln()
+    assert pdf._lasth == 6.35, f"pdf._lasth ({pdf._lasth}) != 5.35 after empty cell"
+    pdf.cell(w=100, txt="Hello world", border=True)
+    assert_pdf_equal(pdf, HERE / "cell_lasth.pdf", tmp_path)