add "mess when resizing" to troubleshooting
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README.md
128
README.md
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@ -1198,6 +1198,133 @@ There are several ways to fix this.
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The parameter you are looking for is called `POWERLEVEL9K_DIR_BACKGROUND`. You can find it in
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in `~/.p10k.zsh`. Uncomment it if it's commented out and try different values.
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### Horrific mess when resizing terminal window
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When you resize terminal window horizontally back and forth a few times, you might see this ugly
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picture.
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tl;dr: This is a bug in Zsh that isn't specific to Powerlevel10k. See [mitigation](#mitigation).
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#### Zsh bug
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This issue is caused by a bug in Zsh that gets triggered the vertical distance between the start of
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the current prompt and the cursor (henceforth `VD`) changes when the terminal window is resized.
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This bug is not specific to Powerlevel10k.
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When a terminal window gets shrunk horizontally, there are two ways for a terminal to handle long
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lines that no longer fit: *reflow* or *truncate*.
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Terminal content before shrinking:
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Terminal reflows text when shrinking:
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Terminal truncates text when shrinking:
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Reflowing strategy can change the hight of terminal content. If such content happens to be between
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the start of the current prompt and the cursor, Zsh will print prompt on the wrong line. Truncation
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strategy never changes the hight of terminal content, so it doesn't trigger this bug in Zsh.
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Let's see how the bug plays out in slow motion. We'll start by launching `zsh -df` and pasting
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the following code:
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```zsh
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function pause() { read -s }
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functions -M pause 0
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reset
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print -l {1..3}
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setopt prompt_subst
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PROMPT=$'${$((pause()))+}left>${(pl.$((COLUMNS-12))..-.)}<right\n> '
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```
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When `PROMPT` gets expanded, it calls `pause` to let us observe the state of the terminal. Here's
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the initial state:
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Zsh keeps track of the cursor position relative to the start of the current prompt. In this case it
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knows that the cursor is one line below. When we shrink the terminal window, it looks like this:
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At this point the terminal sends `SIGWINCH` to Zsh to notify it about changes in the terminal
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dimensions. Note that this signal is sent *after* the content of the terminal has been reflown.
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When Zsh receives `SIGWINCH`, it attempts to erase the current prompt and print it anew. It goes to
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the position where it *thinks* the current prompt is -- one line above the cursor (!) -- erases all
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terminal content that follows and prints reexpanded prompt there. However, after resizing prompt is
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no longer one line above the cursor. It's two lines above! Zsh ends up printing new prompt one line
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too low.
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In this case we ended up with unwanted junk content because `VD` has *increased*. When you make
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terminal window wider, `VD` can also *decrease*, which would result in the new prompt being printed
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higher than intended, potentially erasing useful content in the process.
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Here are a few more examples where shrinking terminal window increased `VD`.
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Simple one-line left prompt with right prompt. No `prompt_subst`. Note that the cursor is below the
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prompt line (hit *ESC-ENTER* to get it there).
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Simple one-line left prompt. No `prompt_subst`, no right prompt. Here `VD` is bound to increase
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upon terminal shrinking due to the command line wrapping around.
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#### Zsh patch
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The bug described above has been fixed in [this branch](
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https://github.com/romkatv/zsh/tree/fix-winchanged). The idea behind the fix is to use `sc` (save
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cursor) terminal capability before printing prompt and `rc` (restore cursor) to move cursor back
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to the same position when prompt needs to be refreshed.
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There are two alternative approaches to fixing the bug that may seem to work at fight glance but in
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fact don't:
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- Instead of `sc`, use `u7` terminal capability to query the current cursor position and then `cup`
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to go back to it. This doesn't work because the absolute position of the start of the current
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prompt changes when text gets reflown.
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- Recompute `VD` based on new terminal dimensions before attempting to refresh prompt. This doesn't
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work because Zsh doesn't know whether terminal reflows text or truncates it. If Zsh could somehow
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know that the terminal reflows text, this approach still wouldn't work on terminals that
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continuously reflow text and rapid-fire `SIGWINCH` when the window is being resized. In such
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environment real terminal dimensions go out of sync with what Zsh thinks the dimensions are.
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There is no ETA for the patch making its way into upstream Zsh. See [discussion](
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https://www.zsh.org/mla/workers//2019/msg00561.html).
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#### Mitigation
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There are a few mitigation options for this issue.
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- Apply [the patch](#zsh-patch) and rebuild Zsh from source.
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- Disable text reflowing on window resize in terminal settings. If your terminal doesn't have this
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setting, try a different terminal.
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- Avoid long lines between the start of prompt and cursor.
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1. Disable right prompt with `POWERLEVEL9K_RIGHT_PROMPT_ELEMENTS=()`.
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2. Disable ruler with `POWERLEVEL9K_SHOW_RULER=false`.
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3. Disable prompt gap with `POWERLEVEL9K_MULTILINE_FIRST_PROMPT_GAP_CHAR=' '`.
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## Table of contents
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- [Features](#features)
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@ -1266,3 +1393,4 @@ There are several ways to fix this.
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- [Weird things happen after typing `source ~/.zshrc`](#weird-things-happen-after-typing-source-zshrc)
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- [Cannot make Powerlevel10k work with my plugin manager](#cannot-make-powerlevel10k-work-with-my-plugin-manager)
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- [Directory is difficult to see in prompt when using Rainbow style](#directory-is-difficult-to-see-in-prompt-when-using-rainbow-style)
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- [Horrific mess when resizing terminal window](#horrific-mess-when-resizing-terminal-window)
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