I've located the problem. I'm using
the imagebox plugin to render wikipedia-style caption boxes around images. If I remove the plugin (which I must do by deleting the directory, as the plugin manager simply takes me back to the wiki start page if I press any of its buttons), then images float correctly. With imagebox, things are broken.
This is also a deal-breaker, as I don't want to give up imagebox, so I'll stay reverted until imagebox is updated, if it ever is.
For reference, though, here are the steps taken to reproduce, and also screenshots and code snippets:
I'm using the 2011-11-29 release of vector, though I upgraded from 2011-05-10. If I try 2011-11-29 on Rincewind, the layout works as expected, with the HRs correctly slicing through the float and confining the images where they're supposed to be. On Angua, I get weirdness.
The problem behavior manifests on Angua after upgrading and deleting the cache directory contents, forcing the pages to be re-rendered.
Here is a screenshot of a page under Rincewind, and
here is the same page under Angua with the problem behavior.
You can view the page yourself, if you like, here:
http://www.bigdinosaur.org/mcwiki/wiki/genesis_base
Here's a snippet of code of one of the image sections displaying bad floating behavior:
==== Notable Features ====
The base consists of several hollowed-out sections inside of Genesis Plateau, as well as several attached structures. The most prominent features are the front facade, the [[wik:barracks]], the [[wiki:genesis_flag|Genesis Flag]], and the developed top of the plateau itself. The two external view pictures provide a good overview of the structure.
[{{:wiki:gb:gb-above-day.jpg?direct&300|Bird's eye view of the base during the day. Rail lines stretch off right to the [[wiki:arcology|Arcology]], bottom to the [[wiki:spawn_point|spawn point]] and [[wiki:ice_hole_valley|Ice Hole Valley]], and left to the [[wiki:cathedral_of_nirin|Cathedral of Nirin]]}}]
[{{:wiki:gb:gb-above-night.jpg?direct&300|The same point of view at night. Note the extent of torches and the lit top of Genesis Plateau and the [[wiki:genesis_flag|flag]] at right, designed as a beacon to help visually locate the base day or night}}]
----
The upper portion of the facade contains a small attic space, inside of which is a small circular powered rail track with a pair of minecarts endlessly looping. This small track serves no real purpose, but hopping on it and riding around in a circle looks hilarious and can be used to great comedic effect. The track was used as the basis for the [[wiki:retard-o-tron|Retard-o-Tron]], inside the main structure.
Just behind the facade is the first of two rail terminuses (termini?), which can be used to launch minecarts to the Cathedral of Nirin, the spawn point, or Ice Hole Valley. Spare minecarts are kept in the chest labeled "Take a minecart, leave a minecart."
[{{:wiki:gb:gb-attic.jpg?direct&300|Entrance to the facade attic}}]
[{{:wiki:gb:gb-railstation.jpg?direct&300|The mini-retard-o-tron}}]
[{{:wiki:gb:gb-attic-rot-test.jpg?direct&300|Rail terminus to the Cathedral of Nirin, the spawn point, or Ice Hole Valley}}]
----
The main entrance opens into a multi-story foyer, with temporary sleeping, crafting, and storage facilities immediately inside for players to use without having to go any deeper into the base. After the door was constructed, these work surfaces were the very first items crafted and placed in the structure. There is a balcony that looks out into the foyer from the library on the second floor, and the moss cobblestone columns are constructed from stone harvested from a series of dungeons that were found inside Genesis Plateau and destroyed.