[LyX/master] Prevent GuiInputMethod from accessing empty cursor slices (#13226)

[Juergen Spitzmueller]

commit 5e8ace434374518502f61822e25df6ccf0dbb637
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Tue Sep 23 14:40:08 2025 +0200

    Prevent GuiInputMethod from accessing empty cursor slices (#13226)
---
 src/frontends/qt/GuiInputMethod.cpp | 44 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/src/frontends/qt/GuiInputMethod.cpp 
b/src/frontends/qt/GuiInputMethod.cpp
index 9655599c7d..869f157936 100644
--- a/src/frontends/qt/GuiInputMethod.cpp
+++ b/src/frontends/qt/GuiInputMethod.cpp
@@ -293,8 +293,37 @@ void GuiInputMethod::processPreedit(QInputMethodEvent* ev)
 
 void GuiInputMethod::onCursorPositionChanged()
 {
+       if (d->cur_->atEnd()) {
+               // Slices are empty:
+               // Reset pos caches and quit.
+               d->cur_pos_ = 0;
+               d->anchor_pos_ = 0;
+               return;
+       }
+
        d->cur_pos_ = d->cur_->top().pos();
-       d->anchor_pos_ = d->cur_->realAnchor().pos();
+
+       // We are only interested in the current paragraph
+       // (where cur_pos_ is) with the anchor_pos_.
+       // Hence:
+       // 1.) If they are at document end, the realAnchor()
+       // slices are empty, so top() [= back()] would assert.
+       // We use the paragraph's last pos.
+       if (d->cur_->realAnchor().atEnd())
+               d->anchor_pos_ = d->cur_->top().lastpos();
+       // 2.) multipar-selection with anchor in later paragraph:
+       // Again, we use the current paragraph's last pos.
+       else if (d->cur_->top().pit() < d->cur_->realAnchor().pit())
+               d->anchor_pos_ = d->cur_->top().lastpos();
+       // 3.) multipar-selection with anchor in previous paragraph
+       // We use pos 0.
+       else if (d->cur_->top().pit() > d->cur_->realAnchor().pit())
+               d->anchor_pos_ = 0;
+       // 4.) singlepar-selection with anchor in same paragraph
+       // Here, finally, we can use the realAnchor().pos()
+       else
+               d->anchor_pos_ = d->cur_->realAnchor().pos();
+
        setSurroundingText(*d->cur_);
 }
 
@@ -1210,6 +1239,10 @@ ParagraphMetrics * 
GuiInputMethod::resetParagraphMetrics(Cursor * cur)
 
 pos_type GuiInputMethod::getCaretPos(size_type preedit_length)
 {
+       if (d->cur_->atEnd())
+               // Slices are empty
+               return 0;
+
        return d->cur_->top().pos() + preedit_length;
 }
 
@@ -1324,8 +1357,17 @@ void GuiInputMethod::setSurroundingText(const Cursor & 
cur) {
 
 void GuiInputMethod::updatePosAndSurroundingText()
 {
+       if (d->cur_->atEnd()) {
+               // Slices are empty:
+               // Reset pos caches and quit.
+               d->cur_pos_ = 0;
+               d->anchor_pos_ = 0;
+               return;
+       }
        if (d->cur_->top().pos() == d->cur_pos_)
+               // Nothing to do
                return;
+
        Q_EMIT cursorPositionChanged();
 }
 
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