Window Tiling doesn't work unless Window Title Bars are at least 2 pixels tall

While testing out Window Themes without a title bar, I found it curious that Window Tiling, by dragging a window to an edge, no longer happens. Tiling using Shortcuts works fine.

On further investigation, once I made the height of the "titlebar" at least two pixels tall it started working again.

The theme I used:

https://github.com/superjamie/onepx

I played around and found that once I changed specifically the file "title-3-active.xpm" to be two pixels tall, that made the difference.

Contents of a working version of said file:

/* XPM */

static char * one_black_pixel[] = { "1 2 1 1", "p c #15539E", "p", "p"};

I also tested an empty theme, that is, one with no .xpm files and an empty themerc, which also didn't tile when dragged.