This bad result was obtained on Debian stable with the 2.8.7.1-1.1+lenny1 version of the libwxgtk2.8-dev package. "make test_c_wxwidgets" illustrates the problem. This target runs the following subset of the examples:
x01c
x04c
x08c
x14c
x16c
x17c
x24c
x30c
The bad horizontal offset title problem shows up for example 1 (see screenshot), example 8, second and subsequent pages (see screenshot), and example 14, second widget page of parent gui. Interestingly, it does not show up for example 16 (which also has multiple pages). One title difference I can see between examples 8 and 16 is 8 uses plmtex while 16 uses pllab. Another difference is 8 uses serif fonts (done by prepending #fr in front of each title) while 16 uses default (sans) fonts.
N.B. screenshots will be attached later.
Scrrenshots will be sent privately because they exceed the minute allowed size limits for this bug tracker.
The new examples/python/test_plplot_encodings.py example was meant to test something else (comparison between UTF-8 and PLPlot encodings for unicode) but also by accident demonstrates the issue in a very simple way. The committed version of the example employs #<sans-serif/> to shift to sans-serif font (which is the default in any case). If you remove that escape sequence from both strings, the horizontal offset problem between the two results disappears. That test proves the issue has something to do with any attempt to do font changes as part of the string with an FCI which should substantially narrow down the search for the bug.