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ConPTY emits nothing to clear the right side when downsizing #12183

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opened 2026-01-31 03:08:18 +00:00 by claunia · 0 comments
claunia commented 2026-01-31 03:08:18 +00:00
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Originally created by @o-sdn-o on GitHub (Jan 25, 2021).

Environment

Windows build number: 10.0.19042.746
Windows Terminal version (if applicable):

Any other software: My own ConPTY-based terminal emulator

I am making my own terminal multiplexer using ConPTY.
ConPTY instances serve as consoles for multiplexer applications. The dimensions of these consoles can be changed by mouse. Each time the console is resized, the ConPTY instance is resized at the same time, which triggers it to redraw its buffer contents.

Using ConPTY in my own terminal emulator, it is impossible to know that visible lines should be truncated when the ConPTY is reduced in size. ConPTY emits nothing to clear the right side when downsizing.
May be related to #5800

Actual behavior

The picture below shows the multiplexer, where the ConPTY consoles are located on the left, and on the right is a window with the output of everything that ConPTY emits when redrawn (top part is raw text, bottom part is code points)

Console lines are doubled in case my built-in terminal wraps the lines by itself:

ConPTY with wrapping

In the case when my built-in terminal has horizontal scrolling enabled, the lines are simultaneously continued and wrapped (since ConPTY wraps lines):

ConPTY without wrapping

It looks like ConPTY does not take into account that the external buffer may be larger and there may be fragments left when the ConPTY is reduced in size.
A possible solution would be to add \e[m\e[K before \r\n at the end of each line to indicate that this is an explicit end of the line.

CSI Ps K Erase in Line (EL), VT100.

Originally created by @o-sdn-o on GitHub (Jan 25, 2021). <!-- 🚨🚨🚨🚨🚨🚨🚨🚨🚨🚨 I ACKNOWLEDGE THE FOLLOWING BEFORE PROCEEDING: 1. If I delete this entire template and go my own path, the core team may close my issue without further explanation or engagement. 2. If I list multiple bugs/concerns in this one issue, the core team may close my issue without further explanation or engagement. 3. If I write an issue that has many duplicates, the core team may close my issue without further explanation or engagement (and without necessarily spending time to find the exact duplicate ID number). 4. If I leave the title incomplete when filing the issue, the core team may close my issue without further explanation or engagement. 5. If I file something completely blank in the body, the core team may close my issue without further explanation or engagement. All good? Then proceed! --> <!-- This bug tracker is monitored by Windows Terminal development team and other technical folks. **Important: When reporting BSODs or security issues, DO NOT attach memory dumps, logs, or traces to Github issues**. Instead, send dumps/traces to secure@microsoft.com, referencing this GitHub issue. If this is an application crash, please also provide a Feedback Hub submission link so we can find your diagnostic data on the backend. Use the category "Apps > Windows Terminal (Preview)" and choose "Share My Feedback" after submission to get the link. Please use this form and describe your issue, concisely but precisely, with as much detail as possible. --> # Environment ```none Windows build number: 10.0.19042.746 Windows Terminal version (if applicable): Any other software: My own ConPTY-based terminal emulator ``` I am making my own terminal multiplexer using ConPTY. ConPTY instances serve as consoles for multiplexer applications. The dimensions of these consoles can be changed by mouse. Each time the console is resized, the ConPTY instance is resized at the same time, which triggers it to redraw its buffer contents. Using ConPTY in my own terminal emulator, it is impossible to know that visible lines should be truncated when the ConPTY is reduced in size. ConPTY emits nothing to clear the right side when downsizing. May be related to #5800 # Actual behavior The picture below shows the multiplexer, where the ConPTY consoles are located on the left, and on the right is a window with the output of everything that ConPTY emits when redrawn (top part is raw text, bottom part is code points) Console lines are doubled in case my built-in terminal wraps the lines by itself: ![ConPTY with wrapping](https://user-images.githubusercontent.com/11535558/105719118-c0fa2a80-5f43-11eb-8c81-d570d8e27d0f.gif) In the case when my built-in terminal has horizontal scrolling enabled, the lines are simultaneously continued and wrapped (since ConPTY wraps lines): ![ConPTY without wrapping](https://user-images.githubusercontent.com/11535558/105719160-cbb4bf80-5f43-11eb-8268-56eadb4fc39d.gif) It looks like ConPTY does not take into account that the external buffer may be larger and there may be fragments left when the ConPTY is reduced in size. A possible solution would be to add `\e[m\e[K` before `\r\n` at the end of each line to indicate that this is an explicit end of the line. [CSI Ps K Erase in Line (EL), VT100.](https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h3-Functions-using-CSI-_-ordered-by-the-final-character_s_)
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Reference: starred/terminal#12183
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