Emacs cannot display the cursor when point is in the middle of a
sequence of text that has the display or composition
property, or is invisible. Therefore, after a command finishes and
returns to the command loop, if point is within such a sequence, the
command loop normally moves point to the edge of the sequence, making this
sequence effectively intangible.
A command can inhibit this feature by setting the variable
disable-point-adjustment:
If this variable is non-nil when a command returns to the
command loop, then the command loop does not check for those text
properties, and does not move point out of sequences that have them.
The command loop sets this variable to nil before each command,
so if a command sets it, the effect applies only to that command.
If you set this variable to a non-nil value, the feature of
moving point out of these sequences is completely turned off.