Inkscape's flowed text implementation supports to render text layout defined with <flowDiv/> elements, even while there is no GUI available to author/write such elements. Two of the files in 'share/examples' also use them (flow-go.svg, flowsample.svg).
The update routines in Inkscape >= 0.92.x to adjust baseline spacing of regular and flowed text in legacy Inkscape SVG files ideally will also support existing Inkscape SVG files with such partially supported elements.
A related test case has been uploaded to this repository, along with PNG exports from several different Inkscape versions for comparison:
https://gitlab.com/su-v/test-files/blob/master/line-spacing/
https://gitlab.com/su-v/test-files/blob/master/line-spacing/wiki_cartography_svg__basic__by_hugolpz-d4q23r0.svg
(the available reduced test cases will be attached to this report as well.)
Attached is a patch based on Tavmjong's feedback on irc (#inkscape-devel) on 2017-02-03:
20:58 Tavmjong : su_v: Line 77 in file-update.cpp should have SP_IS_FLOWDIV(i) added to the 'or'.
which has been updated for the other checks.
Please review the proposed extended patch for any obvious mistakes (C++, logic), and test for any possible regressions.
Scope of the patch:
1) Proposed patch needs to be tracked as 0.92.x task (ideally, to be included in 0.92.1 if accepted by the release manager, else milestoned to 0.92.2).
2) Proposed patch needs "forward"-porting like all other related changes with regard to internal adjustment of baseline spacing in legacy documents on load (not available in lp:inkscape yet).
Updated patch without extra parentheses (cf Mc on irc)