This issue is due to net.minecraft.client.gui.components.EditBox#moveCursorTo testing if the Shift key is down by reading the shiftPressed field, which is not updated properly before moveCursor is called in mouseClicked.
This shiftPressed field is useless because it is private and only used by moveCursorTo and keyPressed, which updates it with a call to net.minecraft.client.gui.screens.Screen#hasShiftDown. keyPressed sets shiftPressed to false before calling deleteText at two places, but this is useless because shiftPressed is only ever read by moveCursor.
Fixing this issue should be as easy as completely removing the shiftPressed field in EditBox, and replacing it in moveCursorTo with a call to Screen#hasShiftDown.
Malo
This issue is due to net.minecraft.client.gui.components.EditBox#moveCursorTo testing if the Shift key is down by reading the shiftPressed field, which is not updated properly before moveCursor is called in mouseClicked.
This shiftPressed field is useless because it is private and only used by moveCursorTo and keyPressed, which updates it with a call to net.minecraft.client.gui.screens.Screen#hasShiftDown. keyPressed sets shiftPressed to false before calling deleteText at two places, but this is useless because shiftPressed is only ever read by moveCursor.
Fixing this issue should be as easy as completely removing the shiftPressed field in EditBox, and replacing it in moveCursorTo with a call to Screen#hasShiftDown.