It turns out that IE11 has a horrible bug whereby setting the
placeholder attribute before setting the text will trigger an input
event:
https://social.msdn.microsoft.com/Forums/ie/en-US/ae4832b0-8eee-4729-b93
3-a9977ea1b583/internet-explorer-input-event-get-fired-when-settingunset
ting-the-placeholder?forum=iewebdevelopment
There have been long standing problems with the buttons not working in
Firefox, and now a [further
problem](https://groups.google.com/d/msg/tiddlywiki/GlsruQyPOag/BAhnI2mt
BgAJ) has come to light, and been similarly hard to resolve.
On balance, I’ve decided to remove the undo/redo buttons from the
toolbar (the ctrl/cmd-Z shortcut is still operational), thus avoiding
the problems, and saving some space in the core.
