I have been investigating Geany's problems with printing.

I looked at
 * Geany 1.22
 * Geany - the current git (as of a few days ago when I started this)
 * Geany 1.23 (git >= 8855c14), en_US.UTF-8
 * SciTE 3.1.0

Geany 1.22 - problems
 * When counting pages, tabs are not expanded, giving inaccurate results - #3475444
 * Printing code mishandles non-monospaced fonts - #2804000
 * When printing, tabs are expanded to fixed width, not to the next tab stop - #2629121.
 * When user requests to print a specified page, page 1 is printed and labeled the requested page.
For example, if the user selects to print page 7 only, Geany will print
page 1, but in the heading, the text will read “Page 7 of xx”. I didn't
submit this as a developer bug, since it is fixed in the current version.

Geany – the current git - problems
 * Print Preview displays a different number of lines than hard-copy - #3580269.
 * The line separating line numbers from text when printing is positioned incorrectly - #3580268.
 * Printed lines are right-shifted compared to previous Geany versions,
causing wrapping which did not occur in previous Geany versions.

SciTE problems
 * Print Preview displays a different number of lines than hard-copy.

The git version of Geany contains a new version of the printing code. This new version uses the printing code contained within Scintilla.

The Scintilla printing code does not draw a vertical line between the line number and the text. So the git version of Geany attempts to draw that line. However, Scintilla provides no method of determining the width of the line number column (SSM(SCI_GETMARGINWIDTHN) returns the display margin width, not the printer margin width). So the Geany code attempts to mimic the calculations Scintilla performs to come up with the column width. The attempt fails, thus bug #3580268.

The Scintilla printing code adds three spaces between line number and text. This shifts the text right, and lines which were nearly page width when printed using the previous versions of Geany now wrap.

As a quick check whether the lines per page mismatch between Print Preview and hard copy is in Geany's code or Scintilla's, I tested SciTE. SciTE has the same problem.

Using the git version of Geany, I printed page 1 of a particular test document, Print Preview listed the document as 18 pages long with 51 lines on the first page (4 lines wrapped) and the hardcopy listed it as 19 pages long with 45 lines on the first page (8 lines wrapped).

Printing the same test with SciTE, Print Preview listed 55 lines on page 1 with 3 lines wrapped, and hardcopy listed 44 lines with 7 lines wrapped. (SciTE's output does not show the number of pages in the document).

I have investigated the problems with the old (1.22 and before) printer code, and I believe all can be addressed by modifying the original code, with the exception of
Printing code mishandles non-monospaced fonts - #2804000

I fixed the bug “When printing, tabs are expanded to fixed width, not to the next tab stop - #2629121” locally a few weeks ago. That was why I joined the developers group. I wanted to submit that patch.

I have not noticed any discrepancy between Print Preview and hardcopy with the (tab expansion patched) old code. And the old code naturally knows the correct location for the vertical line.

When determining the number of pages in the code, the old code currently determines the length of each line by multiplying the width of one character by the number of characters in the line. That is correct as far as UTF8 is concerned – it uses a UTF-aware function to determine number of characters. But it fails to take into consideration the expansion of tabs.

To fix this problem in the old code, one could replace the page counting code. Take the lines of code that actually do the printing, put those lines into a separate function, and call that function twice. First time, the function would calculate the layout (and thus the number of pages) but not actually print. Second time, it prints. If only certain pages are requested, second time it skips lines til a requested page's first line, then prints the page, then skips lines til the next requested page, etc.

Once tabs are expanded correctly, this change seems to fix all the issues except non-monospaced fonts.

An option to deal with that could be to use the new code only for non-monospaced fonts. For monospaced fonts, use the old code. For non-monospaced fonts, the issue of Print Preview not matching hardcopy would remain, but IMO that is a Scintilla bug, and depends on that group for a fix.