Created attachment 53194 [details]
Attached is a test HTML document with two tests related to editable divs that enclose a span that generates content with CSS.
The first test renders the following text:
xxx before span after yyy
Placing the text cursor before the first x and pressing the right arrow key will move the cursor to the right until it reaches the "b" in "before". If the cursor is placed in the word "span", pressing the right arrow key will move the cursor to the right until it reaches (the space before) the "a" in "after". Placing the text cursor after the last y and pressing the left arrow key does move the cursor fluidly through all text. Using [command + right arrow] or [shift + right arrow] will move the text cursor through the CSS-generated content. (However, when using shift the CSS content is selected as if it could be copied, which may be a separate issue.)
The second test renders the following text:
before span after yyy
The only difference is the line break after "xxx". In this case, placing the the text cursor before the first x and pressing the right arrow key will move the cursor onto the next line and into "span". However, using the left arrow key will not move the text cursor back into the previous line. Continuing to press the right arrow key will again move the cursor to the right until it reaches "after".
The last Mac OS X nightly build on which I can reproduce this issue is r97583.
*** This bug has been marked as a duplicate of bug 10123 ***