* lisp/erc/erc.el (erc--initialize-markers): Commit 0d6c8d41ab7 "Use
overlay instead of text prop to hide ERC's prompt" introduced a bug
that caused the prompt to remain hidden upon reconnecting because the
stashed overlay would get clobbered by ERC's major-mode setup.
Binding its old value while unhiding fixes the issue.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Don't permanently set
the default value of `erc-hide-prompt'.
* test/lisp/erc/resources/erc-scenarios-common.el
(erc-scenarios-common--join-network-id): Add assertions for prompt
hiding and unhiding on reconnect.  (Bug#51082)

* lisp/erc/erc-fill.el (erc-fill--wrap-insert-merged-post): Use
correct variable.
* test/lisp/erc/erc-scenarios-display-message.el: Mute unrelated noisy
test.
* test/lisp/erc/erc-tests.el (erc--channel-modes): Bail out following
first group of assertions when running on a graphical display.
(erc--channel-modes/graphic-p): New test asserting different values
for graphical terminals.  Note that this too is tagged as :unstable.
See also: d2ce30afdd2 "; Mark test for erc--update-channel-modes as
:unstable".

* lisp/erc/erc-fill.el (erc-fill-function): Add hyperlink to
`erc-fill-wrap-mode' in doc string.
(erc-fill-wrap-merge-indicator-face): New face.  The rationale for
adding this is that hard-coding `erc-fill-wrap-merge-indicator' to use
specific non-ERC faces forces folks to customize the option by
specifying a value manually, which may not be easy for new users,
seeing as its :type is relatively complex.
(erc-fill-wrap-merge-indicator): Add new preset without leading
space, for narrow windows, and replace `shadow' with
`erc-fill-wrap-merge-indicator-face' everywhere.
(erc-fill-wrap-mode): Add configuration hint to doc string for related
options when facing narrow windows.
(erc-fill--wrap-insert-merged-post, erc-fill--wrap-insert-merged-pre):
Fix sloppy thinko in which "-pre" and "-post" stash variables for
precomputed values were swapped.  Also accommodate :type string
variant for option `erc-fill-wrap-merge-indicator'.
(erc-fill--wrap-rejigger-region): Clear pre-computed merge-indicator
value.  (Bug#60936)

* etc/ERC-NEWS: Add entry for `erc-prompt-format'.
* lisp/erc/erc-compat.el (erc-compat--defer-format-spec-in-buffer):
New macro to wrap `format-spec' specification values in functions that
run in the current buffer and fall back to the empty string.
* lisp/erc/erc.el (erc-prompt): Add predefined Custom choice for
function type in `erc-prompt-format'.
(erc--prompt-format-face-example): New "pre-propertized" value for
option `erc-prompt-format'.
(erc-prompt-format): New companion option for `erc-prompt' choice
`erc-prompt-format'.  New function of the same name to perform format
substitutions and serve as a Custom choice value for `erc-prompt'.
Based on work and ideas originally proposed by Stefan Kangas.
(erc--away-indicator, erc-away-status-indicator,
erc--format-away-indicator): New formatting function and helper
variables for displaying short away status.
(erc--user-modes-indicator): New variable.
(erc--format-user-modes): New function.
(erc--format-channel-status-prefix): New function.
(erc--format-modes): New function.
* test/lisp/erc/erc-scenarios-prompt-format.el: New file.  (Bug#51082)

Co-authored-by: Stefan Kangas <stefankangas@gmail.com>

* lisp/erc/erc-fill.el (erc-fill): Use `when-let' instead of
`when-let*'.
(erc-fill-wrap-merge): Mention companion options in doc string.
(erc-fill-wrap-merge-indicator): New option to display a
distinguishing "indicator" in the form of a one-character string
between messages from the same speaker.
(erc-fill-wrap-mode, erc-fill-wrap-disable): Mention
`erc-fill-wrap-merge-indicator' in doc string and kill related local
variables.
(erc-fill--wrap-merge-indicator-pre,
erc-fill--wrap-merge-indicator-post): New internal variables for
caching merge indicator.
(erc-fill--wrap-insert-merged-post, erc-fill--wrap-insert-merged-pre):
New functions for adding merge indicators either before or after a
message.
(erc-fill-wrap): Add logic for deferring to merge-indicator helpers
when needed.
* test/lisp/erc/erc-fill-tests.el (erc-fill-wrap-tests--merge-action,
erc-fill-wrap--merge-action): Move body of latter test into former, a
new fixture function.
(erc-fill-wrap--merge-action/indicator-pre,
erc-fill-wrap--merge-action/indicator-post): New tests.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-post-01.eld:
New test data file.
* test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-pre-01.eld:
New test data file.  (Bug#60936)

* lisp/erc/erc-fill.el (erc-fill-wrap-align-prompt): New option for
aligning prompt with leading portion of messages at the common "static
center" pivot-column barrier, so it appears "dedented" along with all
speaker name tags.  Tests for this functionality appear in the
subsequent patch of this same change set.
(erc-fill-wrap-use-pixels): Demote from user option to normal variable
because it has no practical use other than for testing.  Don't rename
as internal variable to spare the improbable user of ERC on HEAD who's
already customized this.
(erc-fill-wrap-mode, erc-fill-wrap-enable, erc-fill-wrap-disable):
Take care to disable prompt-in-left-margin behavior when option
`erc-fill-wrap-align-prompt' is non-nil.
(erc-fill--wrap-measure): Improve doc string and always attempt to
leverage `buffer-text-pixel-size', even when the variable
`erc-fill-wrap-use-pixels' is nil.
(erc-fill--wrap-indent-prompt): New function to massage prompt
`line-prefix' after updates, such as changes to away status.
(Bug#51082)

* lisp/erc/erc-backend.el (erc--hidden-prompt-overlay):
New variable, a buffer-local handle for the prompt overlay.
(erc--reveal-prompt): Delete overlay instead of text prop.
(erc--conceal-prompt): Add overlay instead of text prop.
(erc--unhide-prompt): Run `erc--refresh-prompt-hook' after revealing.
(erc--hide-prompt): Run `erc--refresh-prompt-hook' after hiding.
* lisp/erc/erc-stamp.el (erc-stamp--adjust-margin): Attempt a more
accurate estimate of the prompt's width in columns when initially
setting left-margin.
(erc-stamp--skip-left-margin-prompt-p): New variable to inhibit normal
behavior of displaying prompt in left margin.
(erc-stamp--display-margin-mode): Allow opting out of
prompt-in-left-margin behavior.
(erc--reveal-prompt): Delete unneeded method implementation.
(erc--conceal-prompt): Put overlay in margin.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Use
`get-char-property' instead of `get-text-property' in order to
accommodate overlay-based prompt hiding.  (Bug#51082)

* lisp/erc/erc.el (erc--merge-prop-behind-p): New variable to be
dynamically bound around rare calls to `erc--merge-props' when the
latter should append to the end of existing list-valued text
properties.
(erc--inhibit-prompt-display-property-p): New variable to be non-nil
in buffers where an active module needs to reserve all uses of the
`display' text property in the prompt region for itself.
(erc--prompt-properties): Collect all common prompt properties in one
place for code reuse and maintenance purposes.
(erc--refresh-prompt-continue, erc--refresh-prompt-continue-request):
New function and state variable for custom `erc-prompt' functions to
indicate to ERC that they need the prompt to be refreshed in all
buffers and not just the current one.
(erc--refresh-prompt): Merge `erc-prompt-face' behind any applied by a
customized `erc-prompt' function value.  Crucially, don't inherit
properties at the beginning of the prompt because doing so may clobber
any added by a custom `erc-prompt' function.  Instead, apply known
properties from `erc-display-prompt' manually.  Integrate
`erc--refresh-prompt-continue' logic.
(erc--merge-prop): Recognize flag to activate `append' behavior in
which new prop values are appended to the tail of existing ones rather
than consed in front.  This functionality could be extended to
arbitrary splices as well.
(erc-display-prompt): Use common text properties defined elsewhere.
* test/lisp/erc/erc-tests.el (erc--merge-prop): Add assertion for
`erc--merge-prop-behind-p' non-nil behavior.  (Bug#51082)

* lisp/erc/erc-speedbar.el (erc-speedbar-buttons): "Spoof"
`dframe-help-echo-function' in channel buffers to get around the fact
that we're not using the real speedbar frame.
(erc-speedbar--fmt-sentinel): New variable.
(erc-speedbar-expand-channel): Use cached channel-mode string for mode
item.  Use button chars to mark mode and topic items and move verbose
labels into mouse-hover text.  Also set face for mode and topic.
(erc-speedbar-item-info): Look for `speedbar-token' as a fallback even
when the primary pattern doesn't match.  If the value's contents are
structured like a `format' function call, use them.
(erc-speedbar--emulate-sidebar): Add comment about speedbar setting
everything globally in older Emacsen.  (Bug#63595)

* etc/ERC-NEWS: Add entry for more expansive "%m" in header line.
* lisp/erc/erc-common.el (erc--channel-mode-types): New slot
`shortargs' for caching truncated mode args.
* lisp/erc/erc.el (erc--mode-line-chanmodes-arg-len): New internal
variable for adjusting the truncation length of channel-mode arguments
as they appear in the header line.
(erc--mode-line-mode-string): New variable for caching the relevant
"modestring", if any, in ERC buffers.
(erc--process-channel-modes): Don't associate args with group 4/D,
which are all nullary modes.  This fixes a bug in which arguments were
associated with the wrong letters.  Also, set cached mode string for
channel.
(erc--user-modes): Simplify slightly by removing likely useless
variant for overloaded arg AS-TYPE.  This function is new in ERC 5.6.
(erc--channel-modes):  New function.  A higher-level getter for
current channel mode representation to complement `erc--user-modes'.
(erc--parse-user-modes): Set `erc--mode-line-mode-string in server
buffers.
(erc--handle-channel-mode): Change model to associate modes of type A
with a running plus/minus tally of state changes since joining the
channel.
(erc-update-mode-line-buffer): Use cached verbose representation of
channel or user modes instead of calling `erc-format-channel-modes'.
* test/lisp/erc/erc-tests.el (erc--update-channel-modes): Update to
reflect new running tally associations for type A modes.
(erc--channel-modes): New test.
(erc--user-modes): Update to reflect parameter simplification.
(Bug#67220)