In my humble opinion the standard tab width ist far too wide, see attachment. I guess this may be a general problem with GTK3, but it's hard to work, if I can only fit 5 tabs at the same time in the window. Other editors can easily fit twice that much in the same horizontal space.
This problem gets worse when combined with the behavior, that the current tab always will be made moved as much to the right as possible. Meaning when changing tabs the tab bar automatically moves only to the right, but never to the left, making it hard to switch between two tabs, even if they were in the same view. In the screenshot e.g. I would want to switch between tab 3 and tab 6 several times, but as soon as I click on tab 3 tab 6 moves out of view. Yes, there are shortcuts, but I don't always remember the tab numbers and I often have more than 9 tabs open, making these shortcuts unusable.
Actually I'm quite impressed, how easy it is to compile SciTE for GTK2 from the source available here on sourceforge
I wonder why the GTK2-version isn't the default one to be installed from the Debian repo. Maybe I should try a specific XFCE repo ...
Sorry for bothering you, but maybe it is still possible to make the tabs just a tad smaller. Here is how it looks with GTK2. I can fit much more tabs in the half width of my monitor.
Furthermore I noticed just now when comparing both screenshots:
The 'w' of the "View"-menu is cut off by 1 or 2 pixels in the GTK3 version
The tab width is determined by the current system theme. Most themes do not take up so much room horizontally.
Closing as this is a GTK+ theme issue that could be addressed by changing theme.